Statements and Expressions

    if ( condition )
        statement;
    [ else
        statement; ]

(The whole of the preceding example is itself a statement and could be nested within another if/else clause.) The if clause has the common functionality of taking two different forms: a “one-liner” or a block. The block form is as follows:

    if ( condition )  {
        [ statement; ]
        [ statement; ]
        [ ... ]
    } else {
        [ statement; ]
        [ statement; ]
        [ ... ]
    }

    while ( condition )
        statement;
    do
        statement;
    while ( condition );

For example:

    while( queue.isEmpty() )
        wait();

    for ( initialization; condition; incrementor )
        statement;

The variable initialization section can declare or initialize variables that are limited to the scope of the for statement. The for loop then begins a possible series of rounds in which the condition is first checked and, if true, the body statement (or block) is executed. Following each execution of the body, the incrementor expressions are evaluated to give them a chance to update variables before the next round begins:

    for ( int i = 0; i < 100; i++ ) {
        System.out.println( i );
        int j = i;
        ...
    }

This loop will execute 100 times, printing values from 0 to 99. Note that the variable j is local to the block (visible only to statements within it) and will not be accessible to the code “after” the for loop. If the condition of a for loop returns false on the first check, the body and incrementor section will never be executed.

You can use multiple comma-separated expressions in the initialization and incrementation sections of the for loop. For example:

    for (int i = 0, j = 10; i < j; i++, j-- ) {
       ...
    }

    int x;
    for( x = 0; hasMoreValue(); x++ )
      getNextValue();
    System.out.println( x );

    for ( varDeclaration : iterable )
        statement;

    int [] arrayOfInts = new int [] { 1, 2, 3, 4 };

    for( int i  : arrayOfInts )
        System.out.println( i );

    List<String> list = new ArrayList<String>();
    list.add("foo");
    list.add("bar");

    for( String s : list )
        System.out.println( s );

    switch ( expression )
    {
        case constantExpression :
            statement;
        [ case constantExpression :statement;  ]
        ...
        [ default :
            statement;  ]
    }

    int value = 2;

    switch( value ) {
        case 1:
            System.out.println( 1 );
        case 2:
            System.out.println( 2 );
        case 3:
            System.out.println( 3 );
    }

    // prints 2, 3!

Using break to terminate each branch is more common:

    int retValue = checkStatus();

    switch ( retVal )
    {
        case MyClass.GOOD :
            // something good
            break;
        case MyClass.BAD :
            // something bad
            break;
        default :
            // neither one
            break;
    }

In this example, only one branch—GOOD, BAD, or the default—is executed. The “fall through” behavior of the switch is justified when you want to cover several possible case values with the same statement without resorting to a bunch of if/else statements:

    int value = getSize();

    switch( value ) {
        case MINISCULE:
        case TEENYWEENIE:
        case SMALL:
            System.out.println("Small" );
            break;
        case MEDIUM:
            System.out.println("Medium" );
            break;
        case LARGE:
        case EXTRALARGE:
            System.out.println("Large" );
            break;
    }

This example effectively groups the six possible values into three cases.

    enum Size { Small, Medium, Large }

You can use enumerations in switches in the same way that the previous switch examples used integer constants. In fact, it is much safer to do so because the enumerations have real types and the compiler does not let you mistakenly add cases that do not match any value or mix values from different enumerations.

    // usage
    Size size = ...;
    switch ( size ) {
        case Small:
            ...
        case Medium:
            ...
        case Large:
            ...
    }

    while( true ) {
        if ( condition() )
             break;
    }
    // after while

    for( int i=0; i < 100; i++ ) {
        if ( i == 33 )
            continue;
        System.out.println( i );
    }

The break and continue statements look like those in the C language, but Java’s forms have the additional ability to take a label as an argument and jump out multiple levels to the scope of the labeled point in the code. This usage is not very common in day-to-day Java coding, but may be important in special cases. Here is an outline:

    labelOne:
        while ( condition ) {
            ...
            labelTwo:
                while ( condition ) {
                    ...

                    // break or continue point
                }
            // after labelTwo
        }
    // after labelOne

Enclosing statements, such as code blocks, conditionals, and loops, can be labeled with identifiers like labelOne and labelTwo. In this example, a break or continue without argument at the indicated position has the same effect as the earlier examples. A break causes processing to resume at the point labeled “after labelTwo”; a continue immediately causes the labelTwo loop to return to its condition test.

The statement break labelTwo at the indicated point has the same effect as an ordinary break, but break labelOne breaks both levels and resumes at the point labeled “after labelOne.” Similarly, continue labelTwo serves as a normal continue, but continue labelOne returns to the test of the labelOne loop. Multilevel break and continue statements remove the main justification for the evil goto statement in C/C++.

    if (1 < 2)
        return;
    // unreachable statements

    int i, j;          // statement
    i = 5;             // both expression and statement

Normally, we rely on assignment for its side effects alone, but an assignment can be used as a value in another part of an expression:

    j = ( i = 5 );

    int i = myObject.length;
    String s = myObject.name;
    myObject.someMethod();

A reference-type expression can be used in compound evaluations by selecting further variables or methods on the result:

    int len = myObject.name.length();
    int initialLen = myObject.name.substring(5, 10).length();

    System.out.println("calling println on out");

    System.out.println( "Hello, World..." );
    int myLength = myString.length();

Here, we invoked the methods println() and length() on different objects. The length() method returned an integer value; the return type of println() is void (no value).

    Object o = new Object();

    int hours = new Date().getHours();

    Boolean b;
    String str = "foo";
    b = ( str instanceof String ); // true, str is a String
    b = ( str instanceof Object ); // also true, a String is an Object
    //b = ( str instanceof Date ); // The compiler is smart enough to catch this!

instanceof also correctly reports whether the object is of the type of an array or a specified interface (as we’ll discuss later):

    if ( foo instanceof byte[] )
        ...

    String s = null;
    if ( s instanceof String )
        // false, null isn't an instance of anything