Detailed explanation of code examples of Java Object

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Introduction: What are the usage scenarios of hashCode() and equals() in daily coding? How to implement clone deep copy? What do wait() and notify() do? What does the finalize() method do? It seems that it is not used much in coding and is not important, but is there anyone like me who wants to know more about it? After all, it is the foundation of foundations.

The following is a simple and comprehensive summary:

##1. hashCode() and equals()

public boolean equals(Object obj) {return (this == obj);}
public native int hashCode();

1. When the equals() method is overridden, hashCode() will also be overridden.

2. When equals() returns true, the hashcodes must be equal. That is: equal (identical)

objects must have equal hash codes (or hash codes)

3. If the hashCode of two objects is the same, they are not necessarily the same.

4. When searching in a collection, hashcode can greatly reduce the number of

object comparisons and improve search efficiency!

When determining duplicate elements, directly locate the bucket position through the hashcode() method. If there is an element at that position, call the equals() method to determine whether they are equal. Instead of traversing each element and comparing equals()!

2. clone() deep copy

public class Animal implements Cloneable {
  private int height;
  private int age;

  public Animal(int height, int age){
    this.height = height;
    this.age = age;
  }

  @Override
  public Object clone() throws CloneNotSupportedException {
    return super.clone();
  }
}


public class People implements Cloneable {
  private int height;
  private int age;
  private Animal a;

  public People(int height, int age,Animal a){
    this.height = height;
    this.age = age;
    this.a = a;
  }

  @Override
  public Object clone() throws CloneNotSupportedException {
    People p = (People) super.clone();
    p.a = (Animal) a.clone();
    return p;
  }

}

Animal a1 = new Animal(100,3);
People p1 = new People(173,24,a1);
//深复制
People p2 = (People) p1.clone();

3. wait() and notify()

•It can only be called after acquiring the object lock, otherwise an IllegalMonitorStateException will be thrown.

•At any time, the control of the object (monitor) can only be owned by one thread.

There are three ways for a thread to gain control:

1. Execute a synchronized instance method of the object.

2. Execute the synchronization

static method of the corresponding class of the object.

3. Execute the synchronization block that adds a synchronization lock to the object.

Example of executing a synchronization lock on this object:

  synchronized (pepoleObject) {
    pepoleObject.notifyAll();
    pepoleObject.wait();
  }

4. finalize()

When the garbage collector This method is called by the object's garbage collector and may only be called once when it is determined that no more

references exist to the object. When the object is recycled and needs to configure system resources or perform other cleanup, subclasses override the finalize method to implement it.

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