Beginning Java6

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Symbols used for mathematical and logical manipulation that are recognized by the compiler are commonly known as operators in Java. In the first of two lessons on operators we look at the arithmetic, relational, logical and assignment operators.

Arithmetic Operators Top

We are familiar with most of the arithmetic operators in the table below from everyday life and they generally work in the same way in Java. The arithmetic operators work with all the Java numeric primitive types and can also be applied to the char primitive type.

Increment And Decrement Operators

Both these operators can be prepended (prefix) or appended (postfix) to the operand they are used with. When applied to an operand as part of a singular expression it makes no difference whether the increment and decrement operators are applied as a prefix or postfix. With larger expressions however, when an increment or decrement operator precedes its operand, Java will perform the increment or decrement operation prior to obtaining the operand's value for use by the rest of the expression. When an increment or decrement operator follows its operand, Java will perform the increment or decrement operation after obtaining the operand's value for use by the rest of the expression. Lets look at some code to illustrate how this works:

/*
  Increment And Decrement Operators
*/ 
public class IncDec {

    public static void main (String[] args) {
        byte a = 5;
        byte b = 5;
        // Singular Expressions
        a++;
        ++b;
        System.out.println("a = " + a);
        System.out.println("b = " + b);
        // Larger Expressions
        a = 1;
        b = a++;
        System.out.println("b using a postfix = " + b);
        a = 1;
        b = ++a;
        System.out.println("b using a prefix = " + b);
    }

}

Save, compile and run the file in directory   c:\_BeginningJava6

As you can see with the last two lines of output, the postfix increment is applied after the expression is evaluated, whereas the prefix increment is applied before.

Relational Operators Top

Relational Operators refer to the relationships that values can have to each other. Relational Operators produce a true or false result and are used with control statements such as if and while. Any type can be compared for equality or inequality but only types that support an ordering relationship can be applied for comparison. The table below clarifies this.

Logical Operators Top

Logical Operands must be the boolean type and the result of a logical operation is the boolean type and are used with control statements such as if and while. The following table shows all possible combinations and their result.

The short-circuit operators && and || can be more efficient to use; if you want both operands to be evaluated use the & and | operators.

Assignment Operators Top

The single equal sign = is used for assignment in Java and we have been using this throughout the lessons so far. This operator is fairly self explanatory and takes the form variable = expression;. A point to note here is that the type of variable must be compatible with the type of expression.

Shorthand Assignment Operators

The shorthand assignment operators allow us to write compact code that is implemented more efficiently.

Automatic Type Conversion, Assignment Rules Top

The following table shows which types can be assigned to which other types, of course we can assign to the same type so these boxes are greyed out.

When using the table use a row for the left assignment and a column for the right assignment. So in the highlighted permutations byte = int won't convert and int = byte will convert.

Casting Incompatible Types Top

The above table isn't the end of the story though as Java allows us to cast incompatible types. A cast instructs the compiler to convert one type to another enforcing an explicit type conversion.

A cast takes the form    target = (target-type) expression.

There are a couple of things to consider when casting incompatible types:

• With narrowing conversions such as an int to a short there may be a loss of precision if the range of the int exceeds the range of a short as the high order bits will be removed.
• When casting a floating-point type to an integer type the fractional component is lost through truncation.
• The target-type can be the same type as the target or a narrowing conversion type.
• The boolean type is not only incompatible but also inconvertible with other types.

Lets look at some code to see how casting works and the affect it has on values:

/*
  Casting Incompatible Types
*/ 
public class Casting {

    public static void main (String[] args) {
        char a = 'D';
        short b = 129;
        int c = 4127;
        long d = 33445566L;
        float e = 12.34F;
        double f = 456.789;
        byte g = (byte) a;
        System.out.println(a + " When cast from char to byte has the value: " + g); 
        g = (byte) b;
        System.out.println(b + " When cast from short to byte has the value: " + g); 
        g = (byte) c;
        System.out.println(c + " When cast from int to byte has the value: " + g); 
        g = (byte) d;
        System.out.println(d + " When cast from long to byte has the value: " + g); 
        g = (byte) e;
        System.out.println(e + " When cast from float to byte has the value: " + g); 
        g = (byte) f;
        System.out.println(e + " When cast from double to byte has the value: " + g); 
    }
    
}

Save, compile and run the file in directory   c:\_BeginningJava6

The first thing to note is we got a clean compile because of the casts, all the type conversions would fail otherwise. You might be suprised by some of the results shown in the screenshot above, for instance some of the values have become negative. Because we are truncating everything to a byte we are losing not only any fractional components and bits outside the range of a byte, but in some cases the signed bit as well. Casting can be very useful but just be aware of the implications to values when you enforce explicit type conversion.

Operators Quiz Top

Try the quiz below to test your knowledge of this lesson

What's Next?

In our second look at operators we look at the bitwise logical and bitwise shift operators.

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