Java Language Basics

This chapter is from the book

Java Tutorial, The: A Short Course on the Basics, 6th Edition

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This chapter is from the book

This chapter is from the book 

Java Tutorial, The: A Short Course on the Basics, 6th Edition

Learn More Buy

Operators

Now that you’ve learned how to declare and initialize variables, you probably want to know how to do something with them. Learning the operators of the Java programming language is a good place to start. Operators are special symbols that perform specific operations on one, two, or three operands and then return a result.

As we explore the operators of the Java programming language, it may be helpful for you to know ahead of time which operators have the highest precedence. The operators in Table 3.2 are listed according to precedence order. The closer to the top of the table an operator appears, the higher its precedence. Operators with higher precedence are evaluated before operators with relatively lower precedence. Operators on the same line have equal precedence. When operators of equal precedence appear in the same expression, a rule must govern which is evaluated first. All binary operators except for the assignment operators are evaluated from left to right; assignment operators are evaluated right to left.

Table 3.2 Operator Precedence

In general-purpose programming, certain operators tend to appear more frequently than others; for example, the assignment operator (=) is far more common than the unsigned right shift operator (>>>). With that in mind, the following discussion focuses first on the operators that you’re most likely to use on a regular basis and ends focusing on those that are less common. Each discussion is accompanied by sample code that you can compile and run. Studying its output will help reinforce what you’ve just learned.

Assignment, Arithmetic, and Unary Operators

The Simple Assignment Operator

One of the most common operators that you’ll encounter is the simple assignment operator, =. You saw this operator in the Bicycle class; it assigns the value on its right to the operand on its left:

 int cadence = 0;
 int speed = 0;
 int gear = 1;

This operator can also be used on objects to assign object references, as discussed in Chapter 4, “Creating Objects.”

The Arithmetic Operators

The Java programming language provides operators that perform addition, subtraction, multiplication, and division. There’s a good chance you’ll recognize them by their counterparts in basic mathematics. The only symbol that might look new to you is %, which divides one operand by another and returns the remainder as its result.

Table 3.3 Arithmetic Operators

The following program, ArithmeticDemo, tests the arithmetic operators:

class ArithmeticDemo {

    public static void main (String[] args) {
        int result = 1 + 2;
        // result is now 3
        System.out.println("1 + 2 = " + result);
        int original_result = result;

        result = result - 1;
        // result is now 2
        System.out.println(original_result + " - 1 = " + result);
        original_result = result;

        result = result * 2;
        // result is now 4
        System.out.println(original_result + " * 2 = " + result);
        original_result = result;

        result = result / 2;
        // result is now 2
        System.out.println(original_result + " / 2 = " + result);
        original_result = result;

        result = result + 8;
        // result is now 10
        System.out.println(original_result + " + 8 = " + result);
        original_result = result;

        result = result % 7;
        // result is now 3
        System.out.println(original_result + " % 7 = " + result);
    }
}

This program prints the following:

1 + 2 = 3

3 - 1 = 2

2 * 2 = 4

4 / 2 = 2

2 + 8 = 10

10 % 7 = 3

You can also combine the arithmetic operators with the simple assignment operator to create compound assignments. For example, x+=1; and x=x+1; both increment the value of x by 1.

The + operator can also be used for concatenating (joining) two strings together, as shown in the following ConcatDemo program:

class ConcatDemo {
    public static void main(String[] args){
        String firstString = "This is";
        String secondString = " a concatenated string.";
        String thirdString = firstString + secondString;
        System.out.println(thirdString);
    }
}

By the end of this program, the variable thirdString contains "This is a concatenated string.", which gets printed to standard output.

The Unary Operators

The unary operators require only one operand; they perform various operations such as incrementing/decrementing a value by one, negating an expression, or inverting the value of a boolean.

The following program, UnaryDemo, tests the unary operators:

class UnaryDemo {

    public static void main(String[] args) {

        int result = +1;
        // result is now 1
        System.out.println(result);

        result--;
        // result is now 0
        System.out.println(result);

        result++;
        // result is now 1
        System.out.println(result);

        result = -result;
        // result is now -1
        System.out.println(result);

        boolean success = false;
        // false
        System.out.println(success);
        // true
        System.out.println(!success);
    }
}

Table 3.4 Unary Operators

The increment/decrement operators can be applied before (prefix) or after (postfix) the operand. The code result++; and ++result; will both end in result being incremented by one. The only difference is that the prefix version (++result) evaluates to the incremented value, whereas the postfix version (result++) evaluates to the original value. If you are just performing a simple increment/decrement operation, it doesn’t really matter which version you choose. But if you use this operator in part of a larger expression, the one you choose may make a significant difference.

The following program, PrePostDemo, illustrates the prefix/postfix unary increment operator:

class PrePostDemo {
    public static void main(String[] args){
        int i = 3;
        i++;
        // prints 4
        System.out.println(i);
        ++i;
        // prints 5
        System.out.println(i);
        // prints 6
        System.out.println(++i);
        // prints 6
        System.out.println(i++);
        // prints 7
        System.out.println(i);
    }
}

Equality, Relational, and Conditional Operators

The Equality and Relational Operators

The equality and relational operators determine if one operand is greater than, less than, equal to, or not equal to another operand. The majority of these operators will probably look familiar to you as well. Keep in mind that you must use ==, not =, when testing if two primitive values are equal:

== equal to
!= not equal to
> greater than
>= greater than or equal to
< less than
<= less than or equal to

The following program, ComparisonDemo, tests the comparison operators:

class ComparisonDemo {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        if(value1 == value2)
            System.out.println("value1 == value2");
        if(value1 != value2)
            System.out.println("value1 != value2");
        if(value1 > value2)
            System.out.println("value1 > value2");
        if(value1 < value2)
            System.out.println("value1 < value2");
        if(value1 <= value2)
            System.out.println("value1 <= value2");
    }
}

Here is the output:

value1 != value2
value1 < value2
value1 <= value2

The Conditional Operators

The && and || operators perform Conditional-AND and Conditional-OR operations on two boolean expressions. These operators exhibit short-circuiting behavior, which means that the second operand is evaluated only if needed:

&& Conditional-AND
|| Conditional-OR

The following program, ConditionalDemo1, tests these operators:

class ConditionalDemo1 {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        if((value1 == 1) && (value2 == 2))
            System.out.println("value1 is 1 AND value2 is 2");
        if((value1 == 1) || (value2 == 1))

            System.out.println("value1 is 1 OR value2 is 1");
    }
}

Another conditional operator is ?:, which can be thought of as shorthand for an if-then-else statement (discussed in the “Control Flow Statements” section of this chapter). This operator is also known as the ternary operator because it uses three operands. In the following example, this operator should be read as follows: “If someCondition is true, assign the value of value1 to result. Otherwise, assign the value of value2 to result.”

The following program, ConditionalDemo2, tests the ?: operator:

class ConditionalDemo2 {

    public static void main(String[] args){
        int value1 = 1;
        int value2 = 2;
        int result;
        boolean someCondition = true;
        result = someCondition ? value1 : value2;

        System.out.println(result);
    }
}

Because someCondition is true, this program prints 1 to the screen. Use the ?: operator instead of an if-then-else statement if it makes your code more readable (e.g., when the expressions are compact and without side effects, such as in assignments).

The Type Comparison Operator instanceof

The instanceof operator compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

The following program, InstanceofDemo, defines a parent class (named Parent), a simple interface (named MyInterface), and a child class (named Child) that inherits from the parent and implements the interface.

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

        Parent obj1 = new Parent();
        Parent obj2 = new Child();

        System.out.println("obj1 instanceof Parent: "
            + (obj1 instanceof Parent));
        System.out.println("obj1 instanceof Child: "
            + (obj1 instanceof Child));

        System.out.println("obj1 instanceof MyInterface: "
            + (obj1 instanceof MyInterface));
        System.out.println("obj2 instanceof Parent: "
            + (obj2 instanceof Parent));
        System.out.println("obj2 instanceof Child: "
            + (obj2 instanceof Child));
        System.out.println("obj2 instanceof MyInterface: "
            + (obj2 instanceof MyInterface));
    }
}

class Parent {}
class Child extends Parent implements MyInterface {}
interface MyInterface {}

Here is the output:

obj1 instanceof Parent: true
obj1 instanceof Child: false
obj1 instanceof MyInterface: false
obj2 instanceof Parent: true
obj2 instanceof Child: true
obj2 instanceof MyInterface: true

When using the instanceof operator, keep in mind that null is not an instance of anything.

Bitwise and Bit Shift Operators

The Java programming language also provides operators that perform bitwise and bit shift operations on integral types. The operators discussed in this section are less commonly used. Therefore their coverage is brief; the intent is to simply make you aware that these operators exist.

The unary bitwise complement operator (~) inverts a bit pattern; it can be applied to any of the integral types, making every 0 a 1 and every 1 a 0. For example, a byte contains 8 bits; applying this operator to a value whose bit pattern is 00000000 would change its pattern to 11111111.

The signed left shift operator (<<) shifts a bit pattern to the left, and the signed right shift operator (>>) shifts a bit pattern to the right. The bit pattern is given by the left-hand operand, and the number of positions to shift is given by the right-hand operand. The unsigned right shift operator (>>>) shifts a zero into the leftmost position, while the leftmost position after >> depends on sign extension.

The bitwise & operator performs a bitwise AND operation. The bitwise ^ operator performs a bitwise exclusive OR operation. The bitwise | operator performs a bitwise inclusive OR operation.

The following program, BitDemo, uses the bitwise AND operator to print the number 2 to standard output:

class BitDemo {
    public static void main(String[] args) {
        int bitmask = 0x000F;
        int val = 0x2222;
        // prints "2"
        System.out.println(val & bitmask);
    }
}

Summary of Operators

The following quick reference summarizes the operators supported by the Java programming language.

Simple Assignment Operator

Arithmetic Operators

Unary Operators

Equality and Relational Operators

Conditional Operators

Type Comparison Operator

Bitwise and Bit Shift Operators

Questions and Exercises: Operators

Questions

1. Consider the following code snippet:

   arrayOfInts[j] > arrayOfInts[j+1]

   Which operators does the code contain?

2. Consider the following code snippet.

   int i = 10;
   int n = i++%5;

   1. What are the values of i and n after the code is executed?
   2. What are the final values of i and n if instead of using the postfix increment operator (i++), you use the prefix version (++i)?
3. To invert the value of a boolean, which operator would you use?
4. Which operator is used to compare two values, = or == ?
5. Explain the following code sample: result = someCondition ? value1 : value2;

Exercises

1. Change the following program to use compound assignments:

   class ArithmeticDemo {
   
        public static void main (String[] args){
   
             int result = 1 + 2; // result is now 3
             System.out.println(result);
   
             result = result - 1; // result is now 2
             System.out.println(result);
   
             result = result * 2; // result is now 4
             System.out.println(result);
   
             result = result / 2; // result is now 2
             System.out.println(result);
   
             result = result + 8; // result is now 10
             result = result % 7; // result is now 3
             System.out.println(result);
        }
   }

2. In the following program, explain why the value 6 is printed twice in a row:

   class PrePostDemo {
       public static void main(String[] args){
           int i = 3;
           i++;
           System.out.println(i); // "4"
           ++i;
           System.out.println(i); // "5"
           System.out.println(++i); // "6"
           System.out.println(i++); // "6"
           System.out.println(i); // "7"
       }
   }

Answers

You can find answers to these questions and exercises at http://docs.oracle.com/javase/tutorial/java/nutsandbolts/QandE/answers_operators.html.