Java type promotion

In this chapter you will learn:

What is Automatic Type Promotion in Expressions
How to fix the compile-time errors caused by automatic type promotions
What is Java Type Promotion Rules

Automatic Type Promotion in Expressions

For example, examine the following expression:

public class Main {
  public static void main(String[] argv) {
    byte a = 40;//from j ava  2  s . c o m
    byte b = 50;
    byte c = 100;
    int d = a * b / c;
  }
}

The result of a * b exceeds the range of byte. To handle this kind of problem, Java automatically promotes each byte or short operand to int. a * b is performed using integers.

Automatic promotions and compile-time errors

public class Main {
  public static void main(String[] argv) {
    byte b = 5;/*ja  v  a 2  s. c om*/
    b = b * 2; // Error! Cannot assign an int to a byte!
  }
}

Compiling the code above generates the following errors:

If you understand the consequences of overflow, use an explicit cast.

public class Main {
  public static void main(String[] argv) {
    byte b = 50;/*from ja  va2 s .  c  om*/
    b = (byte) (b * 2);

    System.out.println("b is " + b);
  }
}

The output from the code above is:

Type Promotion Rules

Widening conversions do not lose information about the magnitude of a value. For example, an int value is assigned to a double variable. This conversion is legal because doubles are wider than ints. Java's widening conversions are

From a byte to a short, an int, a long, a float, or a double
From a short to an int, a long, a float, or a double
From a char to an int, a long, a float, or a double
From an int to a long, a float, or a double
From a long to a float or a double
From a float to a double

Widening conversions:

char->int
byte->short->int->long->float->double

Here are the Type Promotion Rules:

All byte and short values are promoted to int.
If one operand is a long, the whole expression is promoted to long.
If one operand is a float, the entire expression is promoted to float.
If any of the operands is double, the result is double.

In the following code, f * b, b is promoted to a float and the result of the subexpression is float.

public class Main {
/* ja v  a  2s .  co  m*/
  public static void main(String args[]) {
    byte b = 4;
    float f = 5.5f;
    float result = (f * b);
    System.out.println("f * b = " + result);
  }
}

The output:

In the following program, c is promoted to int, and the result is of type int.

public class Main {
  public static void main(String args[]) {
    char c = 'a';
    int i = 50000;
    int result = i / c;
    System.out.println("i / c is " + result);
  }//  j  av a  2s .c om
}

The output:

In the following code the value of s is promoted to double, and the type of the subexpression is double.

public class Main {
  public static void main(String args[]) {
    short s = 1024;
    double d = .1234;
    double result = d * s;
    System.out.println("d * s is " + result);
  }//from   j  a v a  2 s  .co  m
}

The output:

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What you will learn in the next chapter:

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