Using Built-In Classes

Autoboxing Primitive Types

Autoboxing is the automatic conversion that the Java compiler makes between primitive types and their corresponding object wrapper classes. This includes converting an int to an Integer and a double to a Double.

// These are all examples of autoboxing
Double wrapper1 = 23.456;
Integer wrapper2 = 3;
Double wrapper3 = new Double(13.57);
Integer wrapper4 = new Integer(7);
Double wrapper5 = Double.valueOf(30.59);
Integer wrapper6 = Integer.valueOf(15);

Unboxing Wrapper Classes

Unboxing is the automatic conversion that the Java compiler makes from the wrapper class to the primitive type. This includes converting an Integer to an int and a Double to a double.

Integer IntegerWrapper = new Integer(7);

// Examples of an Integer getting unboxed to an int
int  primitiveInt = IntegerWapper;
int primitiveInt2 = IntegerWrapper.intValue();

Integer and Double Wrapper Classes

The Integer class and Double class are part of the java.lang package. They are used as a wrapper class to allow primitive int and double values to be used by methods that require Objects.

// Primitives
int a = 5;
double b = 2.5;

// Wrapper Objects
Integer c = 7;
Double d = 10.2;

Integer and Double Static Fields

The Integer and Double classes have important static fields named Integer.MAX_VALUE, Integer.MIN_VALUE, Double.POSITIVE_INFINITY, and Double.NEGATIVE_INFINITY. These can be used to find the largest and smallest Integer values that Java can store as well as represent the concept of infinity.

Integer wrapperIntMax = Integer.MAX_VALUE;
Integer wrapperIntMin = Integer.MIN_VALUE;
Double inf = Double.POSITIVE_INFINITY;
Double negInf = Double.NEGATIVE_INFINITY;

Static Methods

Static methods are methods that can be called within a program without creating an object of the class.

// static method
public static int getTotal(int a, int b) {
  return a + b;
}

public static void main(String[] args) {
  int x = 3;
  int y = 2;
  System.out.println(getTotal(x,y)); // Prints: 5
}

Calling a Static Method

Static methods can be called by appending the dot operator to a class name followed by the name of the method.

int largerNumber = Math.max(3, 10); // Call static method
System.out.println(largerNumber); // Prints: 10

The Math Class

The Math class (which is part of the java.lang package) contains a variety of static methods that can be used to perform numerical calculations.

System.out.println(Math.abs(-7.0)); // Prints: 7

System.out.println(Math.pow(5, 3)); // Prints: 125.0

System.out.println(Math.sqrt(52)); // Prints: 7.211102550927978