Java learning notes 18 (Object Class), learning notes object

Java learning notes 18 (Object Class), learning notes object

The Object class is the top-level parent class in Java. All classes are its subclasses and interfaces do not inherit it.

Methods In the Object class:

Official materials: boolean equals (Object obj) indicates whether another Object is "equal" to this Object"

Example:

Package demo; public class Person extends Object {private String name; private int age; public Person () {} public Person (String name, int age) {this. name = name; this. age = age;} // rewrite the equals method of the parent class // the source code is the same, equals is to compare the memory address of the two objects public boolean equals (Object obj) {return this = obj;} 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 ;}}

 

Package demo; public class TestEquals {public static void main (String [] args) {// Person class inherits the Object class, inherited from the parent class method Person p1 = new Person ("Zhang San", 18); Person p2 = new Person ("Li Si", 20); boolean b1 = p1.equals (p2 ); system. out. println (b1); // output false p1 = p2; boolean b2 = p1.equals (p2); System. out. println (b2); // Output true }}

 

However, it is meaningless to compare memory addresses. To meet some requirements, we can rewrite the equals method:

 

Package demo; public class Person extends Object {private String name; private int age; public Person () {} public Person (String name, int age) {this. name = name; this. age = age;} // override the equals method of the parent class, and define the comparison method // compare whether the age of the two objects is the same public boolean equals (Object obj) {// process the passed information. if (this = obj) {return true;} // process the null input. if (obj = null) {return false;} // The subclass feature cannot be called in polymorphism, so you need to perform a downward transformation // Add instanceof to prevent risks, because the Object is a top-level parent class, prevent passing in other classes if (obj instanceof Person) {Person p = (Person) obj; return this. age = p. age;} else {return false ;}} 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 ;}}

Package demo; public class TestEquals {public static void main (String [] args) {// Person class inherits the Object class, inherited from the parent class method Person p1 = new Person ("Zhang San", 20); Person p2 = new Person ("Li Si", 20); boolean b1 = p1.equals (p2 ); system. out. println (b1); // Output true }}

 

ToString method:

Official materials: String toString () returns the String representation of the object

Continue to write an example of the Person class above:

Package demo; public class TestToString {public static void main (String [] args) {Person p1 = new Person ("Zhang San", 18); String s1 = p1.toString (); System. out. println (s1); System. out. println (p1);}/* here the two outputs are the same: demo.Person@2cdb03a1demo.Person @ 2cdb03a1 so we found that if an object is written in the input statement, the toString method of the object is called by default */

 

In fact, obtaining the memory address has no practical significance. To meet some requirements, we rewrite the toString method:

Package demo; public class Person extends Object {private String name; private int age; public Person () {} public Person (String name, int age) {this. name = name; this. age = age;} // override the toString method public String toString () {return this. name + this. 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 ;}}

Package demo; public class TestToString {public static void main (String [] args) {Person p1 = new Person ("Zhang San", 18); String s1 = p1.toString (); System. out. println (s1); System. out. println (p1) ;}}/* The two outputs are always the same: Zhang San, 18 Zhang San, and 18. We found that if an object is written in the input statement, the toString method of the object is always called by default */

 

There are other methods for Object, some of which are not commonly used, and some will be mentioned later.