Method analysis of Object class in Java

Statement

The articles are all my technical notes. Please indicate the source for reprinting https://segmentfault.com/u/yzwall

Introduction to Object class

Object class is the parent class of all classes. In Java, only basic data types are not objects. All array types (object type && basic data type array) are inherited from the Object class;

equals method

In the Object class, two objects are judged by judging whether they have the same reference. Whether the objects are the same;
As long as the subclass overrides the equals method, it must override the hashCode method

// in java.lang.Object
public boolean equals(Object obj) {
    return (this == obj);
}

Principles for rewriting the equals method:

• Reflexivity: A.equals(A) returns true;

• Symmetry: The result of A.equals(B) is sum B.equals(A) is the same;

• Transitiveness: A.equals(B) is true, B.equals(C) is true, then A. equals(C) is true

• Consistency: For non-null references A and B, as long as the object information used in the equals() comparison operation remains unchanged, multiple times Calling A.equals(B), the results are consistent;

• For any non-null reference, x.equals(null) must return false;

• When overriding the equals method, the parameter type must be the Object type

Example of overriding the equals method

class myObject {
    private String name;
    private int age;
    ...
    public getName() {
        return this.name;
    }
    public getAge() {
        return this.age;
    }
}

/**
 * 重写equals方法demo步骤
 * Effective Java中推荐方式
 */
public boolean equals(Object x) {
    // 1. 检查x和this是否引用同一个对象
    if (x == this) {
        return true;
    }
    
    // 2. 检查x对象类型是否是myObject派生
    if (!(x instanceof myObject)) {
        return false;
    }

    // 3. 比较数据域
    // 经过1，2检查，将参数转换为正确类型
    myObject o = (myObject)(x);
    return this.name.equals(x.getName()) && this.age == (x.getAge());
}

hashCode() method

hashCode methodReturns the hash code of the object, Equal objects must return equal hashCode, and the hashCode of different objects are as unequal as possible;

// in java.lang.Object
public native int hashCode();

Rewrite Always rewrite hashCode when equals

• Rewriting equals without rewriting hashCode will result in"unequal objects have the same hashCode", resulting in collection classesHashMap, HashSet and Hashtable will not work; in the extreme case, make the hashCode of all objects equal in the hash table and all objects will be mapped to the same bucket , the hash table degenerates into a linked list;

• When two objects call equal and return true, then the two objects each call hashCode() and return the samehashCode;

• When two objects call equal and return false, the hashCode returned by the two objects each calling hashCode() can be the same (Hash conflicts cannot be completely avoided )

toString() method

The toString method in the Object class outputs the object's "object class name@hash code";

Java Object class method analysis

---

Statement

The articles are all my technical notes. Please indicate the source for reprinting https://segmentfault.com/u/ yzwall

Introduction to Object class

The Object class is the parent class of all classes. In Java, only basic data types are not objects. All array types (object types && basic data type arrays) are inherited from the Object class;

equals method

The Object class determines whether two objects have the same reference. Whether the objects are the same;
As long as the subclass overrides the equals method, it must override the hashCode method

// in java.lang.Object
public boolean equals(Object obj) {
    return (this == obj);
}

Principles for rewriting the equals method:

• Reflexivity: A.equals(A) returns true;

• Symmetry: The result of A.equals(B) is sum B.equals(A) is the same;

• Transitiveness: A.equals(B) is true, B.equals(C) is true, then A. equals(C) is true

• Consistency: For non-null references A and B, as long as the object information used in the equals() comparison operation remains unchanged, multiple times Calling A.equals(B), the results are consistent;

• For any non-null reference, x.equals(null) must return false;

• When overriding the equals method, the parameter type must be the Object type

Example of overriding the equals method

class myObject {
    private String name;
    private int age;
    ...
    public getName() {
        return this.name;
    }
    public getAge() {
        return this.age;
    }
}

/**
 * 重写equals方法demo步骤
 * Effective Java中推荐方式
 */
public boolean equals(Object x) {
    // 1. 检查x和this是否引用同一个对象
    if (x == this) {
        return true;
    }
    
    // 2. 检查x对象类型是否是myObject派生
    if (!(x instanceof myObject)) {
        return false;
    }

    // 3. 比较数据域
    // 经过1，2检查，将参数转换为正确类型
    myObject o = (myObject)(x);
    return this.name.equals(x.getName()) && this.age == (x.getAge());
}

hashCode() method

hashCode methodReturns the hash code of the object, Equal objects must return equal hashCode, and the hashCode of different objects are as unequal as possible;

// in java.lang.Object
public native int hashCode();

Rewrite Always rewrite hashCode when equals

• Rewriting equals without rewriting hashCode will result in"unequal objects have the same hashCode", resulting in collection classesHashMap, HashSet and Hashtable will not work; in the extreme case, make the hashCode of all objects equal in the hash table and all objects will be mapped to the same bucket , the hash table degenerates into a linked list;

• When two objects call equal and return true, then the two objects each call hashCode() and return the samehashCode;

• When two objects call equal and return false, the hashCode returned by the two objects each calling hashCode() can be the same (Hash conflicts cannot be completely avoided )

toString() method

The toString method in the Object class outputs the object's "object class name@hash code";

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