Relational and Conditional Operators - Java Tutorial 5.0 英文版

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Relational and Conditional Operators

A relational operator compares two values and determines the relationship between them. For example, != returns true if its two operands are unequal. The next table summarizes the relational operators.

Relational Operators

Operator	Use	Description
>	op1 > op2	Returns true if op1 is greater than op2
>=	op1 >= op2	Returns true if op1 is greater than or equal to op2
<	op1 < op2	Returns true if op1 is less than op2
<=	op1 <= op2	Returns true if op1 is less than or equal to op2
==	op1 == op2	Returns true if op1 and op2 are equal
!=	op1 != op2	Returns true if op1 and op2 are not equal

The following code is an example, RelationalDemo, that defines three integer numbers and uses the relational operators to compare them. The relational operations are shown in boldface.

public class RelationalDemo {
    public static void main(String[] args) {

        //a few numbers
        int i = 37;
        int j = 42;
        int k = 42;
        System.out.println("Variable values...");
        System.out.println("   i = " + i);
        System.out.println("   j = " + j);
        System.out.println("   k = " + k);

        //greater than
        System.out.println("Greater than...");
        System.out.println("   i > j = " + (i > j)); //false
        System.out.println("   j > i = " + (j > i)); //true
        System.out.println("   k > j = " + (k > j)); //false
                                            //(they are equal)

        //greater than or equal to
        System.out.println("Greater than or equal to...");
        System.out.println("   i >= j = " + (i >= j)); //false
        System.out.println("   j >= i = " + (j >= i)); //true
        System.out.println("   k >= j = " + (k >= j)); //true

        //less than
        System.out.println("Less than...");
        System.out.println("   i < j = " + (i < j)); //true
        System.out.println("   j < i = " + (j < i)); //false
        System.out.println("   k < j = " + (k < j)); //false

        //less than or equal to
        System.out.println("Less than or equal to...");
        System.out.println("   i <= j = " + (i <= j)); //true
        System.out.println("   j <= i = " + (j <= i)); //false
        System.out.println("   k <= j = " + (k <= j)); //true

        //equal to
        System.out.println("Equal to...");
        System.out.println("   i == j = " + (i == j)); //false
        System.out.println("   k == j = " + (k == j)); //true

        //not equal to
        System.out.println("Not equal to...");
        System.out.println("   i != j = " + (i != j)); //true
        System.out.println("   k != j = " + (k != j)); //false
    }
}

Here's the output from this program.

Variable values...
   i = 37
   j = 42
   k = 42
Greater than...
   i > j = false
   j > i = true
   k > j = false
Greater than or equal to...
   i >= j = false
   j >= i = true
   k >= j = true
Less than...
   i < j = true
   j < i = false
   k < j = false
Less than or equal to...
   i <= j = true
   j <= i = false
   k <= j = true
Equal to...
   i == j = false
   k == j = true
Not equal to...
   i != j = true
   k != j = false

Relational operators often are used with conditional operators to construct more complex decision-making expressions. The Java programming language supports six conditional operators — five binary and one unary — as shown in the following table.

Conditional Operators

Operator	Use	Description
&&	op1 && op2	Returns true if op1 and op2 are both true; conditionally evaluates op2
||	op1 || op2	Returns true if either op1 or op2 is true; conditionally evaluates op2
!	!op	Returns true if op is false
&	op1 & op2	Returns true if op1 and op2 are both boolean and both true; always evaluates op1 and op2; if both operands are numbers, performs bitwise AND operation
|	op1 | op2	Returns true if both op1 and op2 are boolean and either op1 or op2 is true; always evaluates op1 and op2; if both operands are numbers, performs bitwise inclusive OR operation
^	op1 ^ op2	Returns true if op1 and op2 are different — that is, if one or the other of the operands, but not both, is true

One such operator is &&, which performs the conditional AND operation. You can use two different relational operators along with && to determine whether both relationships are true. The following line of code uses this technique to determine whether an array index is between two boundaries. It determines whether the index is both greater than or equal to 0 and less than NUM_ENTRIES, which is a previously defined constant value.

0 <= index && index < NUM_ENTRIES

Note that, in some instances, the second operand to a conditional operator may not be evaluated. Consider this code segment.

(numChars < LIMIT) && (...)

The && operator returns true only if both operands are true. So, if numChars is greater than or equal to LIMIT, the left operand for && is false, and the return value of && can be determined without evaluating the right operand. In such a case, the interpreter does not evaluate the right operand. This case has important implications if the right operand has side effects, such as reading from a stream, updating a value, or making a calculation.

When both operands are boolean, the operator & performs the same operation as &&. However, & always evaluates both of its operands and returns true if both are true. Likewise, when the operands are boolean, | performs the same operation as ||. The | operator always evaluates both of its operands and returns true if at least one of its operands is true. When their operands are numbers, & and | perform bitwise manipulations. The next section contains more information on this.

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