[Java Review] 8: Object class

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The following notes are based on video series: 手把手教你用Java基础教程 - 北京尚学堂 - 马士兵

Java API specfication: http://www.oracle.com/technetwork/java/api-141528.html

I. Object

  • Object is the root class of all classes. ex.

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    public class <classnanme>{}

    is equal to 

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    public class <classnanme> extends Object{}

  • toString():

    • when connector is used, this super method is auto-executed.
    • return “\<classname>@\<hashcode>“.
    • usually overriden.

  • equals():

    • if using ==, it is actually comparing ref. address.

    • default is same as ==

    • should be overriden, ex:

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      public boolean equals(Object obj){
          if (obj == null) return false;
          else{
              if (obj instanceof Cat){
                  Cat c = (Cat) obj;
                  if (c.color ==this color && c.height == this.height){
                      return true;
                  }
              }
          }
          return false;
      }
II. Casting

  • A superclass ref. can point to its subclass instance.

  • For above, it can not access new member variables in its subclass.

  • instanceof can be used to check if an instance belongs to a class or superclass.

  • upcasting: treat subclass as superclass;

    downcasting: treat superclass as subclass;

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//sample application 
public class Test{
    public static void main(String args[]){
        Test test = new Test();
        Animal a = new Animal("name");
        Cat c = new Cat("catname", "blue");
        Dog d = new Dog("dogname", "black");
        test.f(a);
        test.f(c);
        test.f(d);
	}
    
    public void f(Animal a){
        System.out.println("name: " + a.name);
        if (a instanceof Cat){
            Cat cat = (Cat)a;
            System.out.println("eyecolor: " + cat.eyecolor);
        } else if (a instanceof Dog) {
            Dog dog = (Dog)a;
            System.out.println("furcolor: " + dog.furcolor);
        }
    }
}
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