Java Tutorial/Class Definition/Class Object

Assignment with objects is a bit tricky.

class Number {
  int i;
}
public class MainClass {
  public static void main(String[] args) {
    Number n1 = new Number();
    Number n2 = new Number();
    n1.i = 9;
    n2.i = 47;
    System.out.println("1: n1.i: " + n1.i + ", n2.i: " + n2.i);
    n1 = n2;
    System.out.println("2: n1.i: " + n1.i + ", n2.i: " + n2.i);
    n1.i = 27;
    System.out.println("3: n1.i: " + n1.i + ", n2.i: " + n2.i);
  }
}

1: n1.i: 9, n2.i: 47
2: n1.i: 47, n2.i: 47
3: n1.i: 27, n2.i: 27

Class Object

1. All the classes defined are subclasses by default.
2. Object is a superclass of every class.
3. The inheritance happens automatically.
4. Your classes will inherit members from the class Object.

MethodPurposetoString()returns a string describing the current objectequals()compares two objectsgetClass()returns a Class that identifies the class of the current object.hashCode()calculates a hashcode for an objectnotify()wakes up a thread associated with the current object.notifyAll()wakes up all threads associated with the current object.wait()causes a thread to wait

class Dog{
  public Dog(String aType){
  }
}
public class MainClass{
  public static void main(String[] a){
    Dog d = new Dog("a");
    Class objectType = d.getClass();             
    System.out.println(objectType.getName());      
  }
}

Dog

Demonstrate Run-Time Type Information.

class X {
  int a;
  float b;
}
class Y extends X {
  double c;
}
class MainClass {
  public static void main(String args[]) {
    X x = new X();
    Y y = new Y();
    Class<?> clObj;
    clObj = x.getClass(); 
    System.out.println("x is object of type: " + clObj.getName());
    clObj = y.getClass(); 
    System.out.println("y is object of type: " + clObj.getName());
    
    clObj = clObj.getSuperclass();
    System.out.println("y"s superclass is " + clObj.getName());
  }
}

Getting java.lang.Class: Information about your object

public class MainClass {
  public static void main(String[] a) {
    String country = "Canada";
    Class myClass = country.getClass();
    System.out.println(myClass.getName());
  }
}

java.lang.String

Storing a reference to the String object as type Object

A variable of type Object can store a reference to an object of any class type.

public class MainClass {
  public static void main(String[] a) {
    String saying = "A stitch in time saves nine.";
    Object str = saying;
    System.out.println(str);
    System.out.println(str.getClass().getName());
  }
}

A stitch in time saves nine.
java.lang.String

The "Class" class also brings the possibility of creating an object without using the new keyword.

1. The static forName method creates a Class object of the given class name.
2. The newInstance method creates a new instance of a class.

public class MainClass {
  public static void main(String[] a) {
    Class klass = null;
    try {
      klass = Class.forName("java.lang.String");
    } catch (ClassNotFoundException e) {
    
    }
    
    if (klass != null) {
      try {
        // create an instance of the Test class
        String test = (String) klass.newInstance();

      } catch (IllegalAccessException e) {
      } catch (InstantiationException e) {
      }
    }
  }
}