This specification is not final and is subject to change. Use is subject to license terms.

Pattern matching for instanceof

Changes to the Java® Language Specification • Version 14-internal+0-adhoc.dlsmith.20191211

This document describes changes to the Java Language Specification to support pattern matching in instanceof expressions, a preview feature of Java SE 14. See JEP 305 for an overview of the feature.

Changes are described with respect to existing sections of the JLS. New text is indicated like this and deleted text is indicated like this. Explanation and discussion, as needed, is set aside in grey boxes.

Chapter 1: Introduction

1.1 Organization of the Specification

...

Chapter 14 describes blocks and statements, which are based on C and C++, and patterns, which provide a means for describing the shape of data. The language has no goto statement, but includes labeled break and continue statements. Unlike C, the Java programming language requires boolean (or Boolean) expressions in control-flow statements, and does not convert types to boolean implicitly (except through unboxing), in the hope of catching more errors at compile time. A synchronized statement provides basic object-level monitor locking. A try statement can include catch and finally clauses to protect against non-local control transfers.

...

Chapter 4: Types, Values, and Variables

4.11 Where Types Are Used

Types are used in most kinds of declaration and in certain kinds of expression. Specifically, there are 16 17 type contexts where types are used:

Also, types are used as:

Finally, there are three special terms in the Java programming language which denote the use of a type:

...

4.12 Variables

4.12.3 Kinds of Variables

There are eight nine kinds of variables:

  1. A class variable is a field declared using the keyword static within a class declaration (8.3.1.1), or with or without the keyword static within an interface declaration (9.3).

    A class variable is created when its class or interface is prepared (12.3.2) and is initialized to a default value (4.12.5). The class variable effectively ceases to exist when its class or interface is unloaded (12.7).

  2. An instance variable is a field declared within a class declaration without using the keyword static (8.3.1.1).

    If a class T has a field a that is an instance variable, then a new instance variable a is created and initialized to a default value (4.12.5) as part of each newly created object of class T or of any class that is a subclass of T (8.1.4). The instance variable effectively ceases to exist when the object of which it is a field is no longer referenced, after any necessary finalization of the object (12.6) has been completed.

  3. Array components are unnamed variables that are created and initialized to default values (4.12.5) whenever a new object that is an array is created (10, 15.10.2). The array components effectively cease to exist when the array is no longer referenced.

  4. Method parameters (8.4.1) name argument values passed to a method.

    For every parameter declared in a method declaration, a new parameter variable is created each time that method is invoked (15.12). The new variable is initialized with the corresponding argument value from the method invocation. The method parameter effectively ceases to exist when the execution of the body of the method is complete.

  5. Constructor parameters (8.8.1) name argument values passed to a constructor.

    For every parameter declared in a constructor declaration, a new parameter variable is created each time a class instance creation expression (15.9) or explicit constructor invocation (8.8.7) invokes that constructor. The new variable is initialized with the corresponding argument value from the creation expression or constructor invocation. The constructor parameter effectively ceases to exist when the execution of the body of the constructor is complete.

  6. Lambda parameters (15.27.1) name argument values passed to a lambda expression body (15.27.2).

    For every parameter declared in a lambda expression, a new parameter variable is created each time a method implemented by the lambda body is invoked (15.12). The new variable is initialized with the corresponding argument value from the method invocation. The lambda parameter effectively ceases to exist when the execution of the lambda expression body is complete.

  7. An exception parameter is created each time an exception is caught by a catch clause of a try statement (14.20).

    The new variable is initialized with the actual object associated with the exception (11.3, 14.18). The exception parameter effectively ceases to exist when execution of the block associated with the catch clause is complete.

  8. Local variables are declared by local variable declaration statements (14.4).

    Whenever the flow of control enters a block (14.2) or for statement (14.14), a new variable is created for each local variable declared in a local variable declaration statement immediately contained within that block or for statement.

    A local variable declaration statement may contain an expression which initializes the variable. The local variable with an initializing expression is not initialized, however, until the local variable declaration statement that declares it is executed. (The rules of definite assignment (16) prevent the value of a local variable from being used before it has been initialized or otherwise assigned a value.) The local variable effectively ceases to exist when the execution of the block or for statement is complete.

    Were it not for one exceptional situation, a local variable could always be regarded as being created when its local variable declaration statement is executed. The exceptional situation involves the switch statement (14.11), where it is possible for control to enter a block but bypass execution of a local variable declaration statement. Because of the restrictions imposed by the rules of definite assignment (16), however, the local variable declared by such a bypassed local variable declaration statement cannot be used before it has been definitely assigned a value by an assignment expression (15.26).

  1. Pattern variables are declared in patterns. Pattern variables are assigned a value by the process of pattern matching (14.30.3). This process is conditional; a pattern variable is only assigned a value if the pattern match succeeds. For this reason, pattern variables require special rules restricting their use (6.3).

Example 4.12.3-1. Different Kinds of Variables

class Point {
    static int numPoints;   // numPoints is a class variable
    int x, y;               // x and y are instance variables
    int[] w = new int[10];  // w[0] is an array component
    int setX(int x) {       // x is a method parameter
        int oldx = this.x;  // oldx is a local variable
        this.x = x;
        return oldx;
    }
    boolean equalAtX(Object o) {
        if (o instanceof Point p)  // p is a pattern variable
            return this.x == p.x;
        else
            return false;
    }
}

4.12.4 final Variables

A variable can be declared final. A final variable may only be assigned to once. It is a compile-time error if a final variable is assigned to unless it is definitely unassigned immediately prior to the assignment (16).

Once a final variable has been assigned, it always contains the same value. If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object. This applies also to arrays, because arrays are objects; if a final variable holds a reference to an array, then the components of the array may be changed by operations on the array, but the variable will always refer to the same array.

A blank final is a final variable whose declaration lacks an initializer.

A constant variable is a final variable of primitive type or type String that is initialized with a constant expression (15.28). Whether a variable is a constant variable or not may have implications with respect to class initialization (12.4.1), binary compatibility (13.1), reachability (14.21), and definite assignment (16.1.1).

Three Four kinds of variable are implicitly declared final: a field of an interface (9.3), a local variable declared as a resource of a try-with-resources statement (14.20.3), and an exception parameter of a multi-catch clause (14.20), and a pattern variable (14.30.1). An exception parameter of a uni-catch clause is never implicitly declared final, but may be effectively final.

Example 4.12.4-1. Final Variables

Declaring a variable final can serve as useful documentation that its value will not change and can help avoid programming errors. In this program:

class Point {
    int x, y;
    int useCount;
    Point(int x, int y) { this.x = x; this.y = y; }
    static final Point origin = new Point(0, 0);
}

the class Point declares a final class variable origin. The origin variable holds a reference to an object that is an instance of class Point whose coordinates are (0, 0). The value of the variable Point.origin can never change, so it always refers to the same Point object, the one created by its initializer. However, an operation on this Point object might change its state - for example, modifying its useCount or even, misleadingly, its x or y coordinate.

Certain variables that are not declared final are instead considered effectively final:

If a variable is effectively final, adding the final modifier to its declaration will not introduce any compile-time errors. Conversely, a local variable or parameter that is declared final in a valid program becomes effectively final if the final modifier is removed.

4.12.5 Initial Values of Variables

Every variable in a program must have a value before its value is used:

Example 4.12.5-1. Initial Values of Variables

class Point {
    static int npoints;
    int x, y;
    Point root;
}

class Test {
    public static void main(String[] args) {
        System.out.println("npoints=" + Point.npoints);
        Point p = new Point();
        System.out.println("p.x=" + p.x + ", p.y=" + p.y);
        System.out.println("p.root=" + p.root);
    }
}

This program prints:

npoints=0
p.x=0, p.y=0
p.root=null

illustrating the default initialization of npoints, which occurs when the class Point is prepared (12.3.2), and the default initialization of x, y, and root, which occurs when a new Point is instantiated. See 12 for a full description of all aspects of loading, linking, and initialization of classes and interfaces, plus a description of the instantiation of classes to make new class instances.

Chapter 5: Conversions and Contexts

5.5 Casting Contexts

Casting contexts allow the operand of a cast expression (15.16) to be converted to the type explicitly named by the cast operator, and the first operand of the instanceof operator (15.20.2) to be converted to the type indicated by the second operand. Compared to assignment contexts and invocation contexts, casting contexts allow the use of more of the conversions defined in 5.1, and allow more combinations of those conversions.

...

Chapter 6: Names

6.1 Declarations

A declaration introduces an entity into a program and includes an identifier (3.8) that can be used in a name to refer to this entity. The identifier is constrained to be a type identifier when the entity being introduced is a class, interface, or type parameter.

A declared entity is one of the following:

Constructors (8.8) are also introduced by declarations, but use the name of the class in which they are declared rather than introducing a new name.

6.3 Scope of a Declaration

The scope of a declaration is the region of the program within which the entity declared by the declaration can be referred to using a simple name, provided it is not shadowed (6.4.1).

A declaration is said to be in scope at a particular point in a program if and only if the declaration's scope includes that point.

The scope of the declaration of an observable top level package (7.4.3) is all observable compilation units associated with modules to which the package is uniquely visible (7.4.3).

The declaration of a package that is not observable is never in scope.

The declaration of a subpackage is never in scope.

The package java is always in scope.

The scope of a type imported by a single-type-import declaration (7.5.1) or a type-import-on-demand declaration (7.5.2) is the module declaration (7.7) and all the class and interface type declarations (7.6) of the compilation unit in which the import declaration appears, as well as any annotations on the module declaration or package declaration of the compilation unit.

The scope of a member imported by a single-static-import declaration (7.5.3) or a static-import-on-demand declaration (7.5.4) is the module declaration and all the class and interface type declarations of the compilation unit in which the import declaration appears, as well as any annotations on the module declaration or package declaration of the compilation unit.

The scope of a top level type (7.6) is all type declarations in the package in which the top level type is declared.

The scope of a declaration of a member m declared in or inherited by a class type C (8.1.6) is the entire body of C, including any nested type declarations.

The scope of a declaration of a member m declared in or inherited by an interface type I (9.1.4) is the entire body of I, including any nested type declarations.

The scope of an enum constant C declared in an enum type T is the body of T, and any case label of a switch statement whose expression is of enum type T (14.11).

The scope of a formal parameter of a method (8.4.1), constructor (8.8.1), or lambda expression (15.27) is the entire body of the method, constructor, or lambda expression.

The scope of a class's type parameter (8.1.2) is the type parameter section of the class declaration, the type parameter section of any superclass or superinterface of the class declaration, and the class body.

The scope of an interface's type parameter (9.1.2) is the type parameter section of the interface declaration, the type parameter section of any superinterface of the interface declaration, and the interface body.

The scope of a method's type parameter (8.4.4) is the entire declaration of the method, including the type parameter section, but excluding the method modifiers.

The scope of a constructor's type parameter (8.8.4) is the entire declaration of the constructor, including the type parameter section, but excluding the constructor modifiers.

The scope of a local class declaration immediately enclosed by a block (14.2) is the rest of the immediately enclosing block, including its own class declaration.

The scope of a local class declaration immediately enclosed by a switch block statement group (14.11) is the rest of the immediately enclosing switch block statement group, including its own class declaration.

The scope of a local variable declaration in a block (14.4) is the rest of the block in which the declaration appears, starting with its own initializer and including any further declarators to the right in the local variable declaration statement.

The scope of a local variable declared in the ForInit part of a basic for statement (14.14.1) includes all of the following:

The scope of a local variable declared in the FormalParameter part of an enhanced for statement (14.14.2) is the contained Statement.

The scope of a parameter of an exception handler that is declared in a catch clause of a try statement (14.20) is the entire block associated with the catch.

The scope of a variable declared in the ResourceSpecification of a try-with-resources statement (14.20.3) is from the declaration rightward over the remainder of the ResourceSpecification and the entire try block associated with the try-with-resources statement.

The translation of a try-with-resources statement implies the rule above.

Example 6.3-1. Scope of Type Declarations

These rules imply that declarations of class and interface types need not appear before uses of the types. In the following program, the use of PointList in class Point is valid, because the scope of the class declaration PointList includes both class Point and class PointList, as well as any other type declarations in other compilation units of package points.

package points;
class Point {
    int x, y;
    PointList list;
    Point next;
}

class PointList {
    Point first;
}

Example 6.3-2. Scope of Local Variable Declarations

The following program causes a compile-time error because the initialization of local variable x is within the scope of the declaration of local variable x, but the local variable x does not yet have a value and cannot be used. The field x has a value of 0 (assigned when Test1 was initialized) but is a red herring since it is shadowed (6.4.1) by the local variable x.

class Test1 {
    static int x;
    public static void main(String[] args) {
        int x = x;
    }
}

The following program does compile:

class Test2 {
    static int x;
    public static void main(String[] args) {
        int x = (x=2)*2;
        System.out.println(x);
    }
}

because the local variable x is definitely assigned (16) before it is used. It prints:

4

In the following program, the initializer for three can correctly refer to the variable two declared in an earlier declarator, and the method invocation in the next line can correctly refer to the variable three declared earlier in the block.

class Test3 {
    public static void main(String[] args) {
        System.out.print("2+1=");
        int two = 2, three = two + 1;
        System.out.println(three);
    }
}

This program produces the output:

2+1=3

A variable declared in a pattern is known as a pattern variable (14.30). Pattern variables differ from other local variables in that they can only be assigned a value by pattern matching (14.30.3). This process is conditional; a pattern variable is only assigned a value if the pattern match succeeds.

Accordingly the scope of pattern variables is carefully defined so that a pattern variable is only in scope at those program points where pattern matching will have succeeded and the pattern variable will have been definitely assigned a value. Put another way, accessing a pattern variable where pattern matching can not be guaranteed to have succeeded is not possible and will result in a compile-time error.

In this sense, the scope of a pattern variable is a flow dependent concept similar to definite assignment (Chapter 16). The rules that are defined in the rest of this section deliberately have a similar form to those used in Chapter 16.

The scope of a pattern variable is determined by considering the innermost enclosing statement S that contains the pattern variable declaration. The overall scope of a pattern variable V is defined to be (i) those expressions and statements contained in S where V is definitely matched; and (ii) if S is immediately contained by a statement Q, those statements following S contained by Q where V is definitely matched; and (iii) if S is immediately contained by a block, those statements following S contained by that block where V is definitely matched.

The remainder of this section is devoted to a precise explanation of the words "definitely matched at", for which we define three auxiliary technical terms:

The analysis takes into account the structure of statements and expressions, with a special treatment for the boolean expression operators and certain statement forms.

The simplest example is that the pattern variable s is introduced by the expression a instanceof String s when true. In other words, if the value of the expression is true then the pattern matching must have succeeded, and thus the pattern variable must have been assigned a value.

In contrast, the pattern variable t is introduced by the expression !(b instanceof Integer t) when false. This is because the pattern matching could only have succeeded if the value of the expression is false.

Pattern variables can, in certain circumstances, be introduced by a statement. Further details are given in 6.3.2.

6.3.1 Pattern Declaration Scopes and Expressions

Only certain boolean expressions can introduce a new pattern variable into scope. If an expression is not a logical complement expression, conditional-and expression, conditional-or expression, conditional expression, or instanceof operator, then no rules apply regarding the introduction of pattern variables.

6.3.1.1 Conditional-And Operator &&

The following rules apply to a conditional-and expression (15.23):

The first rule rules out the possibility of a pattern variable being implicitly assigned a value more than once by pattern matching. Pattern variables are implicitly final by design.

if ((a instanceof String s) && (b instanceof String s)) {
    System.out.println(s);   // Not allowed
}

The second rule means that a pattern variable introduced by the left-hand operand of a conditional-and operator is in scope, and can therefore be used, in the right-hand operand. This allows for expressions such as x instanceof String s && s.length()>0.

6.3.1.2 Conditional-Or Operator ||

The following rules apply to a conditional-or expression (15.24):

6.3.1.3 Logical Complement Operator !

The following rules apply to a logical complement expression (15.15.6):

6.3.1.4 Conditional Operator ? :

The following rules apply to a conditional expression a ? b : c (15.25):

6.3.1.5 instanceof Operator

The following rule applies to an instanceof expression (15.20.2):

Note that no pattern variable is introduced by an expression a instanceof p when false.

JEP 361 proposes to add a switch expression. This would result in the following additional rule for determining the scope of pattern variables (in conjunction with the changes required to specify switch expressions).

6.3.1.6 switch Expressions

The following rule covers the switch expression (15.28).

  • A pattern variable introduced by a statement S contained in a switch labeled statement group (14.11.1) is definitely matched at all the statements following S, if any, in the switch labeled statement group.

6.3.2 Pattern Declaration Scopes and Statements

Only a few statements play a significant role in determining the scope of pattern variables.

The scope of pattern variables declared in subexpressions contained within if, while, do, and for statements can, in certain circumstances, include other contained substatements. Here is an example:

if (x instance String s) {
    // String s in scope for this block
    // No explicit cast needed here!
    ...
    System.out.println("The string value was: " + s);
} else {
    // String s not in scope here
    System.out.println(s); // Compile-time error!
}

In certain constrained circumstances, a pattern variable can be introduced by a statement. In this case, the pattern variable is in scope at following statements in the enclosing block. Here is an example:

public void RequiresAString(Object o) {
    if (!(o instanceof String s)) {
        throw new IllegalArgumentException();
    }
    // Only reachable if the pattern match succeeded
    // String s is thus in scope for the rest of the block
    System.out.println("The parameter string was: " + s);
    ...
}
6.3.2.1 Blocks

The following rule applies to a block statement S contained in a block that is not a switch block:

6.3.2.2 if Statements

The following rules apply to a statement if (e) S (14.9.1):

The second rule makes use of the notion of 'cannot complete normally' (14.21), which itself makes use of the concept of a constant expression (15.29). This means that calculating the scope of a pattern variable may require determining whether a simple name, or a qualified name of the form TypeName. Identifier, refers to a constant variable. As pattern variables can never refer to a constant variable, there is no circularity.

The following rules apply to a statement if (e) S else T (14.9.2):

These rules highlight the flow-like nature of scoping for pattern variables. In the following statement:

if (e instanceof String s) {
  counter += s.length();
} else {
  ...   // s not in scope
}

The pattern variable s is introduced by the instanceof operator and is in scope in the first contained statement (the one before the else keyword), but it is not in scope in the second contained statement (the one after the else keyword).

Moreover, combined with the treatment for the boolean expressions, the scope of pattern variables is robust against code refactorings that exploit the familar boolean logical equivalences. For example, the previous code can be rewritten as:

if (!(e instanceof String s)) {
  ...   // s not in scope
} else {
  counter += s.length();
}

And, furthermore, can be rewritten as:

if (!!(e instanceof String s)) {
  counter += s.length();
} else {
  ...   // s not in scope
}
6.3.2.3 while Statements

The following rules apply to a statement while (e) S (14.12):

6.3.2.4 do Statements

The following rule applies to a statement do S while (e) (14.13):

6.3.2.5 for Statements

The following rules cover the basic for statement (14.14.1). Since the enhanced for statement (14.14.2) is defined by translation to a basic for statement, no special rules need to be provided for it.

6.3.2.6 switch Statements

The following rule covers the switch statement (14.11).

6.4 Shadowing and Obscuring

A local variable (14.4), formal parameter (8.4.1, 15.27.1), exception parameter (14.20), and local class (14.3), and pattern variable (14.30) can only be referred to using a simple name, not a qualified name (6.2).

Some declarations are not permitted within the scope of a local variable, pattern variable, formal parameter, exception parameter, or local class declaration because it would be impossible to distinguish between the declared entities using only simple names.

For example, if the name of a formal parameter of a method could be redeclared as the name of a local variable in the method body, then the local variable would shadow the formal parameter and there would be no way to refer to the formal parameter - an undesirable outcome.

It is a compile-time error if the name of a formal parameter is used to declare a new variable within the body of the method, constructor, or lambda expression, unless the new variable is declared within a class declaration contained by the method, constructor, or lambda expression.

It is a compile-time error if the name of a local variable v is used to declare a new variable within the scope of v, unless the new variable is declared within a class whose declaration is within the scope of v.

It is a compile-time error if the name of an exception parameter is used to declare a new variable within the Block of the catch clause, unless the new variable is declared within a class declaration contained by the Block of the catch clause.

It is a compile-time error if the name of a local class C is used to declare a new local class within the scope of C, unless the new local class is declared within another class whose declaration is within the scope of C.

These rules allow redeclaration of a variable or local class in nested class declarations that occur in the scope of the variable or local class; such nested class declarations may be local classes (14.3) or anonymous classes (15.9). Thus, the declaration of a formal parameter, local variable, pattern variable, or local class may be shadowed in a class declaration nested within a method, constructor, or lambda expression; and the declaration of an exception parameter may be shadowed in a class declaration nested within the Block of the catch clause.

There are two design alternatives for handling name clashes created by lambda parameters and other variables declared in lambda expressions. One is to mimic class declarations: like local classes, lambda expressions introduce a new "level" for names, and all variable names outside the expression can be redeclared. Another is a "local" strategy: like catch clauses, for loops, and blocks, lambda expressions operate at the same "level" as the enclosing context, and local variables outside the expression cannot be shadowed. The above rules use the local strategy; there is no special dispensation that allows a variable declared in a lambda expression to shadow a variable declared in an enclosing method.

Note that the rule for local classes does not make an exception for a class of the same name declared within the local class itself. However, this case is prohibited by a separate rule: a class cannot have the same name as a class that encloses it (8.1).

Example 6.4-1. Attempted Shadowing Of A Local Variable

Because a declaration of an identifier as a local variable of a method, constructor, or initializer block must not appear within the scope of a parameter, or local variable, or pattern variable of the same name, a compile-time error occurs for the following program:

class Test1 {
    public static void main(String[] args) {
        int i;
        for (int i = 0; i < 10; i++)
            System.out.println(i);
    }
}

This restriction helps to detect some otherwise very obscure bugs. A similar restriction on shadowing of members by local variables was judged impractical, because the addition of a member in a superclass could cause subclasses to have to rename local variables. Related considerations make restrictions on shadowing of local variables by members of nested classes, or on shadowing of local variables by local variables declared within nested classes unattractive as well.

Hence, the following program compiles without error:

class Test2 {
    public static void main(String[] args) {
        int i;
        class Local {
            {
                for (int i = 0; i < 10; i++)
                    System.out.println(i);
            }
        }
        new Local();
    }
}

On the other hand, local variables with the same name may be declared in two separate blocks or for statements, neither of which contains the other:

class Test3 {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++)
            System.out.print(i + " ");
        for (int i = 10; i > 0; i--)
            System.out.print(i + " ");
        System.out.println();
    }
}

This program compiles without error and, when executed, produces the output:

0 1 2 3 4 5 6 7 8 9 10 9 8 7 6 5 4 3 2 1

This style is also common with pattern matching, where repeated type test patterns often employ the same pattern name:

class Test4 {
    class Point {
        int x, y;
    }
    public static void test(Object a, Object b, Object c) {
        if (a instanceof Point p) {
            System.out.print("a is a point (" + p.x + ", " + p.y);
        }
        if (b instanceof Point p){
            System.out.print("b is a point (" + p.x + ", " + p.y);
        } else if (c instanceof Point p) {
            System.out.print("c is a point (" + p.x + ", " + p.y);
        }
        System.out.println();
    }
}

However, pattern variables are not allowed to shadow other pattern variables; a compile-time error occurs for the following program:

class Test5 {
    public static void test(Object a, Object b, Object c) {
        if (a instanceof Point p) {
            System.out.print("a is a point (" + p.x + ", " + p.y);
            if (b instanceof Point p){
                System.out.print("b is a point (" + p.x + ", " + p.y);
            }
        }
        System.out.println();
    }
}

Pattern variables are, like local variables, not allowed to shadow local variables; a compile-time error occurs for the following program:

class Test6 {
    class Point {
        int x, y;
        Point(int x, int y) {
            this.x = x;
            this.y = y;
        }
    }
    public static void test(Object o) {
        Point p = new Point(0,0);
        if (o instanceof Point p)
            System.out.println("I get your point");
    }
}

6.4.1 Shadowing

Some declarations may be shadowed in part of their scope by another declaration of the same name, in which case a simple name cannot be used to refer to the declared entity.

Shadowing is distinct from hiding (8.3, 8.4.8.2, 8.5, 9.3, 9.5), which applies only to members which would otherwise be inherited but are not because of a declaration in a subclass. Shadowing is also distinct from obscuring (6.4.2).

A declaration d of a type named n shadows the declarations of any other types named n that are in scope at the point where d occurs throughout the scope of d.

A declaration d of a field or formal parameter named n shadows, throughout the scope of d, the declarations of any other variables named n that are in scope at the point where d occurs.

A declaration d of a local variable, pattern variable, or exception parameter named n shadows, throughout the scope of d, (a) the declarations of any other fields named n that are in scope at the point where d occurs, and (b) the declarations of any other variables named n that are in scope at the point where d occurs but are not declared in the innermost class in which d is declared.

A declaration d of a method named n shadows the declarations of any other methods named n that are in an enclosing scope at the point where d occurs throughout the scope of d.

A package declaration never shadows any other declaration.

A type-import-on-demand declaration never causes any other declaration to be shadowed.

A static-import-on-demand declaration never causes any other declaration to be shadowed.

A single-type-import declaration d in a compilation unit c of package p that imports a type named n shadows, throughout c, the declarations of:

A single-static-import declaration d in a compilation unit c of package p that imports a field named n shadows the declaration of any static field named n imported by a static-import-on-demand declaration in c, throughout c.

A single-static-import declaration d in a compilation unit c of package p that imports a method named n with signature s shadows the declaration of any static method named n with signature s imported by a static-import-on-demand declaration in c, throughout c.

A single-static-import declaration d in a compilation unit c of package p that imports a type named n shadows, throughout c, the declarations of:

Example 6.4.1-1. Shadowing of a Field Declaration by a Local Variable Declaration

class Test {
    static int x = 1;
    public static void main(String[] args) {
        int x = 0;
        System.out.print("x=" + x);
        System.out.println(", Test.x=" + Test.x);
    }
}

This program produces the output:

x=0, Test.x=1

This program declares:

Since the scope of a class variable includes the entire body of the class (8.2), the class variable x would normally be available throughout the entire body of the method main. In this example, however, the class variable x is shadowed within the body of the method main by the declaration of the local variable x.

A local variable has as its scope the rest of the block in which it is declared (6.3); in this case this is the rest of the body of the main method, namely its initializer "0" and the invocations of System.out.print and System.out.println.

This means that:

The keyword this can also be used to access a shadowed field x, using the form this.x. Indeed, this idiom typically appears in constructors (8.8):

class Pair {
    Object first, second;
    public Pair(Object first, Object second) {
        this.first = first;
        this.second = second;
    }
}

Here, the constructor takes parameters having the same names as the fields to be initialized. This is simpler than having to invent different names for the parameters and is not too confusing in this stylized context. In general, however, it is considered poor style to have local variables with the same names as fields.

Example 6.4.1-2. Shadowing of a Type Declaration by Another Type Declaration

import java.util.*;
class Vector {
    int val[] = { 1 , 2 };
}

class Test {
    public static void main(String[] args) {
        Vector v = new Vector();
        System.out.println(v.val[0]);
    }
}

The program compiles and prints:

1

using the class Vector declared here in preference to the generic class java.util.Vector (8.1.2) that might be imported on demand.

6.5 Determining the Meaning of a Name

6.5.1 Syntactic Classification of a Name According to Context

...

A name is syntactically classified as an ExpressionName in these contexts:

...

6.5.2 Reclassification of Contextually Ambiguous Names

An AmbiguousName is then reclassified as follows.

If the AmbiguousName is a simple name, consisting of a single Identifier:

...

6.5.6 Meaning of Expression Names

6.5.6.1 Simple Expression Names

If an expression name consists of a single Identifier, then there must be exactly one declaration denoting either a local variable, formal parameter, or field in scope at the point at which the Identifier occurs. Otherwise, a compile-time error occurs.

If the declaration denotes an instance variable (8.3.1.1), the expression name must appear within an instance method (8.4.3.2), instance variable initializer (8.3.2), instance initializer (8.6), or constructor (8.8). If the expression name appears within a class method, class variable initializer, or static initializer (8.7), then a compile-time error occurs.

If the declaration declares a final variable which is definitely assigned before the simple expression, the meaning of the name is the value of that variable. Otherwise, the meaning of the expression name is the variable declared by the declaration.

If the expression name appears in an assignment context, invocation context, or casting context, then the type of the expression name is the declared type of the field, local variable, pattern variable, or parameter after capture conversion (5.1.10).

Otherwise, the type of the expression name is the declared type of the field, local variable, pattern variable, or parameter.

That is, if the expression name appears "on the right hand side", its type is subject to capture conversion. If the expression name is a variable that appears "on the left hand side", its type is not subject to capture conversion.

Example 6.5.6.1-1. Simple Expression Names

class Test {
    static int v;
    static final int f = 3;
    public static void main(String[] args) {
        int i;
        i = 1;
        v = 2;
        f = 33;  // compile-time error
        System.out.println(i + " " + v + " " + f);
    }
}

In this program, the names used as the left-hand-sides in the assignments to i, v, and f denote the local variable i, the field v, and the value of f (not the variable f, because f is a final variable). The example therefore produces an error at compile time because the last assignment does not have a variable as its left-hand side. If the erroneous assignment is removed, the modified code can be compiled and it will produce the output:

1 2 3

Chapter 9: Interfaces

9.6 Annotation Types

9.6.4 Predefined Annotation Types

9.6.4.1 @Target

An annotation of type java.lang.annotation.Target is used on the declaration of an annotation type T to specify the contexts in which T is applicable. java.lang.annotation.Target has a single element, value, of type java.lang.annotation.ElementType[], to specify contexts.

Annotation types may be applicable in declaration contexts, where annotations apply to declarations, or in type contexts, where annotations apply to types used in declarations and expressions.

There are nine declaration contexts, each corresponding to an enum constant of java.lang.annotation.ElementType:

  1. Module declarations (7.7)

    Corresponds to java.lang.annotation.ElementType.MODULE

  2. Package declarations (7.4.1)

    Corresponds to java.lang.annotation.ElementType.PACKAGE

  3. Type declarations: class, interface, enum, and annotation type declarations (8.1.1, 9.1.1, 8.5, 9.5, 8.9, 9.6)

    Corresponds to java.lang.annotation.ElementType.TYPE

    Additionally, annotation type declarations correspond to java.lang.annotation.ElementType.ANNOTATION_TYPE

  4. Method declarations (including elements of annotation types) (8.4.3, 9.4, 9.6.1)

    Corresponds to java.lang.annotation.ElementType.METHOD

  5. Constructor declarations (8.8.3)

    Corresponds to java.lang.annotation.ElementType.CONSTRUCTOR

  6. Type parameter declarations of generic classes, interfaces, methods, and constructors (8.1.2, 9.1.2, 8.4.4, 8.8.4)

    Corresponds to java.lang.annotation.ElementType.TYPE_PARAMETER

  7. Field declarations (including enum constants) (8.3.1, 9.3, 8.9.1)

    Corresponds to java.lang.annotation.ElementType.FIELD

  8. Formal and exception parameter declarations (8.4.1, 9.4, 14.20)

    Corresponds to java.lang.annotation.ElementType.PARAMETER

  9. Local variable declarations (including loop variables of for statements, and resource variables of try-with-resources statements, and pattern variables) (14.4, 14.14.1, 14.14.2, 14.20.3, 15.20.2)

    Corresponds to java.lang.annotation.ElementType.LOCAL_VARIABLE

There are 16 type contexts (4.11), all represented by the enum constant TYPE_USE of java.lang.annotation.ElementType.

It is a compile-time error if the same enum constant appears more than once in the value element of an annotation of type java.lang.annotation.Target.

If an annotation of type java.lang.annotation.Target is not present on the declaration of an annotation type T, then T is applicable in all declaration contexts except type parameter declarations, and in no type contexts.

These contexts are the syntactic locations where annotations were allowed in Java SE 7.

Chapter 14: Blocks and Statements and Patterns

Sections 14.22-14.29 are left deliberately unused to allow for future language evolution.

14.30 Patterns

A pattern describes the shape of data. Pattern matching is the process of comparing a value against a pattern and determining whether the value matches the pattern or not. A pattern may in addition declare a pattern variable to name a component of the shape. If a value matches against a pattern, then this variable is assigned a value by the process of pattern matching. The treatment of scoping of pattern variables in 6.3.1 ensures that pattern variables are only in scope where matching is guaranteed to have succeeded and hence the pattern variable will be bound with a value at run-time.

14.30.1 Kinds of Patterns

Pattern:
TypeTestPattern
14.30.1.1 Type Test Pattern
TypeTestPattern:
ReferenceType Identifier

A type test pattern consists of a type and a pattern variable. It is a compile-time error if the type does not denote a reference type (4.3).

The type of a type test pattern is the ReferenceType.

A type test pattern is said to declare the pattern variable Identifier. The scope of that pattern variable Identifier is conditional on the context, as defined in 6.3. The type of the pattern variable Identifier is defined to be ReferenceType.

14.30.2 Compatibility of an Expression with a Pattern

An expression is compatible with a pattern as follows:

At compile time, the instanceof operator (15.20.2) checks compatibility of its first operand, an expression, with the type of its second operand.

14.30.3 Execution of Pattern Matching

At run time, a value is matched against a pattern. If the value matches the pattern, then additionally values may be assigned to any pattern variables declared in the pattern. The rules for determining whether a value matches a pattern (or not) are as follows:

All other possibilities are excluded by the compile time type-checking of pattern matching.

Chapter 15: Expressions

15.20 Relational Operators

The numerical comparison operators <, >, <=, and >=, and the instanceof operator, are called the relational operators.

RelationalExpression:
ShiftExpression
RelationalExpression < ShiftExpression
RelationalExpression > ShiftExpression
RelationalExpression <= ShiftExpression
RelationalExpression >= ShiftExpression
RelationalExpression instanceof ReferenceType ReferenceTypeOrPattern

Patterns are defined in 14.30.

The relational operators are syntactically left-associative (they group left-to-right).

However, this fact is not useful. For example, a<b<c parses as (a<b)<c, which is always a compile-time error, because the type of a<b is always boolean and < is not an operator on boolean values.

The type of a relational expression is always boolean.

15.20.2 Type Comparison Operator instanceof The instanceof Operator

The type of the RelationalExpression operand of the instanceof operator must be a reference type or the null type, or a compile-time error occurs.

It is a compile-time error if the ReferenceType mentioned after the instanceof operator does not denote a reference type that is reifiable (4.7).

If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error (15.16), then the instanceof relational expression likewise produces a compile-time error. In such a situation, the result of the instanceof expression could never be true.

At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

ReferenceTypeOrPattern:
ReferenceType
Pattern

An instanceof operator has one of two forms: (i) a type instanceof operator, where the ReferenceTypeOrPattern operand is a ReferenceType; or (ii) a pattern instanceof operator, where the ReferenceTypeOrPattern operand is a Pattern.

The following applies to a type instanceof operator:

The following applies to a pattern instanceof operator:

Example 15.20.2-1. The type instanceof Operator

class Point   { int x, y; }
class Element { int atomicNumber; }
class Test {
    public static void main(String[] args) {
        Point   p = new Point();
        Element e = new Element();
        if (e instanceof Point) {  // compile-time error
            System.out.println("I get your point!");
            p = (Point)e;  // compile-time error
        }
    }
}

This program results in two compile-time errors. The cast (Point)e is incorrect because no instance of Element or any of its possible subclasses (none are shown here) could possibly be an instance of any subclass of Point. The instanceof expression is incorrect for exactly the same reason. If, on the other hand, the class Point were a subclass of Element (an admittedly strange notion in this example):

class Point extends Element { int x, y; }

then the cast would be possible, though it would require a run-time check, and the instanceof expression would then be sensible and valid. The cast (Point)e would never raise an exception because it would not be executed if the value of e could not correctly be cast to type Point.