Abstract Methods

An abstract method has no body or implementation code.

public abstract void changeBulb();    // Abstract method
public void changeBulb();             // ILLEGAL: This method requires a body instead of a semicolon
public abstract void changeBulb() { } // ILLEGAL: abstract methods do not specify a body

Concrete subclasses must implement all inherited abstract methods of their superclass.

• A subclass that fails to do so must itself be declared abstract.

public abstract class LEDLight extends AbstractLight {
}

An abstract class can define non-abstract, or concrete, methods.

public abstract class AbstractLight {
  public abstract void changeBulb();  // Abstract method

  private int lumens;
  public int getLumens() {            // Concrete method
    return lumens;
  }
  public void setLumens(int lumens) { // Concrete method
    this.lumens = lumens;
  }
}

* Subclasses can override these (unless they are declared final or private), but are not required to.

Practice Exercise

The numeric wrapper classes extend the abstract class Number, which defines both abstract and concrete methods:

public abstract class Number implements java.io.Serializable {
    public abstract int intValue();
    public abstract long longValue();
    public abstract float floatValue();
    public abstract double doubleValue();
    public byte byteValue() {
        return (byte)intValue();
    }
    public short shortValue() {
        return (short)intValue();
    }
  //...
}

An abstract class is not required to declare any abstract methods.

• It can be subclassed, but not instantiated.

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