Class Object

Object class is the parent class of all Java class types (or ancestor class is more suitable)
The first chapter of Thinking in Java is called "everything is Object". Everything is an object.

toString() method: Self Description

Any class inherits Object by default, so there are toString() methods:

public class Test {
    public static void main(String[] args) {
        Test a = new Test();
        System.out.println(a.toString());
        System.out.println(a);
    }
}

When printing an Object, we call the toString() method of this class by default. Even if there is no toString() code in the definition of this class, we will inherit this method from the Object.

The toString() method in the Object class is defined as:

public String toString() {
    return getClass().getName() + "@" + Integer.toHexString(hashCode());
}

The default string is "class name @ hashCode".

Rewrite toString to realize self description

class Person {
    public Person(String name) {
        this.name = name;
    }
    private String name;
    @Override
    public String toString() {
        return getClass().getName() + "{" + name + "}";
    }
}
public class TestToString {
    public static void main(String[] args) {
        Person a = new Person("Monkey King");
        System.out.println(a.toString());
    }
}

Result:

Person {sunwalker}

Practical application example:

toString() can be generated automatically: right click source Generate toString()

public class Monkey {
    private int id;
    private String type;
    private String name;
    // Accessor slightly
    @Override
    public String toString() {
        return "Monkey [id=" + id + ", type=" + type + ", name=" + name + "]";
    }
}

equals method and==

There is no difference between the equals method and the = = operator in the Object class (two Object references are equal to be considered equal):

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

The String class overrides the equals method, so content equality is determined to be equal.

When you define a class, you can override the equals method to provide a customized judgment standard.

// For example: two classes with the same name are considered equal
public class TestOverwriteEquals {
    public static void main(String[] args) {
        Food f1 = new Food("rice");
        Food f2 = new Food("rice");
        System.out.println("==:" + (f1 == f2));
        System.out.println("equals:" + f1.equals(f2));
    }
}
class Food {
    public Food(String name) {
        this.name = name;
    }
    private String name;
    @Override
    public boolean equals(Object obj) {
        if (obj != null && obj.getClass() == Food.class) {
            Food f = (Food) obj;
            if (f.name.equals(this.name)) {
                return true;
            }
        }
        return false;
    }
}

Operation result

==:false
equals:true