Object class

• Superclass, base class, direct or indirect parent class of all classes, located at the top of the inheritance tree.
• If any class does not write extends to inherit a class, it will inherit 0bject class directly by default, otherwise it will inherit indirectly.
• The method defined in 0bject class is the method that all objects have.
• 0object type can store any object.
  ● as a parameter, any object can be accepted.
  ● as a return value, any object can be returned.

Common methods of Object

getClass() method

• public final Class<?> getClass() {}
• Returns the actual object type stored in the reference.
• Application: it is usually used to judge whether the actual storage object types in two references are consistent.

public class Test01 {
    private String name;
    private int age;

    public Test01(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public static void main(String[] args) {
        Test01 test01 = new Test01("zheng",23);
        Test01 test02 = new Test01("li",22);
        Class class1 = test01.getClass();
        Class class2 = test02.getClass();
        if (class1 == class2){
            System.out.println("Is the same type");
        }else {
            System.out.println("Are different types");
        }
    }
}

result:

hashCode() method

• public int hashCode() {}
• Returns the hash code value of the object.
• Hash value is the number of int types calculated by hash algorithm according to the address or string or number of the object
  Value.
• Generally, the same object returns the same hash code.

System.out.println(test01.hashCode());
System.out.println(test02.hashCode());

toString() method

• public String toString() {}
• Returns the string representation (representation) of the object.
• This method can be overridden according to program requirements, such as displaying the attribute values of the object.

System.out.println(test01.toString());
System.out.println(test02.toString());

equals() method

• public boolean equals (0bject obj) {}
• The default implementation is (this == obj). Compare whether the addresses of two objects are the same.
• You can overwrite and compare whether the contents of the two objects are the same.

System.out.println(test01.equals(test02));

The equals() method overrides the steps

• Compare whether two references point to the same object.
• Judge whether obj is null.
• Judge whether the actual object types pointed to by the two references are consistent.
• Cast type.
• Compare whether the attribute values are the same in turn.

 @Override
    public boolean equals(Object obj) {
        if (this == obj){
            return true;
        }
        if (obj == null){
            return false;
        }
        if (obj instanceof Test01){
            Test01 test01 = (Test01)obj;
            if (this.name.equals(test01.getName())&&this.age == test01.getAge()){
                return true;
            }
        }
        return false;
    }

finalize() method

• When the object is determined to be a garbage object, the JVM will automatically call this method to mark the garbage object and enter the collection queue.
• Garbage object: garbage object when there is no valid reference to this object.
• Garbage collection: GC destroys garbage objects to free up data storage space.
• Automatic recycling mechanism: the JVM runs out of memory and recycles all garbage objects at one time.
• Manual recycling mechanism: use system gc(); Notify the JVM to perform garbage collection.

 new Test01("aaa",26);
 new Test01("bbb",26);
 new Test01("ccc",26);
 System.gc();