2.11 Operators

Java supports almost all of the standard C operators. These standard operators have the same precedence and associativity in Java as they do in C. They are listed in Table 2.3 and also in quick reference form in Chapter 13, Java Syntax.

Note the following Java operator differences from C. Java does not support the comma operator for combining two expressions into one (although the for statement simulates this operator in a useful way). Since Java does not allow you to manipulate pointers directly, it does not support the reference and dereference operators *, ->, and &, nor the sizeof operator. Further, Java doesn't consider [] (array access) and . (field access) to be operators, as C does.

Java also adds some new operators:

The + operator applied to String values concatenates them. [5] If only one operand of + is a String, the other one is converted to a string. The conversion is done automatically for primitive types, and by calling the toString() method of non-primitive types. This String + operator has the same precedence as the arithmetic + operator. The += operator works as you would expect for String values.

[5] To C++ programmers, this looks like operator overloading. In fact, Java does not support operator overloading--the language designers decided (after much debate) that overloaded operators were a neat idea, but that code that relied on them became hard to read and understand.

The instanceof operator returns true if the object o on its left-hand side is an instance of the class C or implements the interface I specified on its right-hand side. It also returns true if o is an instance of a subclass of C or is an instance of a subclass of some class that implements I. instanceof returns false if o is not an instance of C or does not implement I. It also returns false if the value on its left is null. If instanceof returns true, it means that o is assignable to variables of type C or I. The instanceof operator has the same precedence as the <, <=, >, and >= operators.

Because all integral types in Java are signed values, the Java >> operator is defined to do a right shift with sign extension. The >>> operator treats the value to be shifted as an unsigned number and shifts the bits right with zero extension. The >>>= operator works as you would expect.

& and |

When & and | are applied to integral types in Java, they perform the expected bitwise AND and OR operations. Java makes a strong distinction between integral types and the boolean type, however. Thus, if these operators are applied to boolean types, they perform logical AND and logical OR operations. These logical AND and logical OR operators always evaluate both of their operands, even when the result of the operation is determined after evaluating only the left operand. This is useful when the operands are expressions with side effects (such as method calls) and you always want the side effects to occur. However, when you do not want the right operand evaluated if it is not necessary, you can use the && and || operators, which perform "short-circuited" logical AND and logical OR operations just as in C. The &= and |= operators perform a bitwise or logical operation depending on the type of the operands, as you would expect.