[JAVA] Difference between abstract and interface

I was embarrassed that I couldn't answer the Java language specification at the interview, so I will relearn it.

About the relationship between interface and abstract

Expressed in various ways.

# interface abstract
Specifications Used when you want to define as a class specification Used when there is an inheritance relationship and you want to reuse the process
Relationship with implementation class or concrete class Implementation classCANAbstract function Concrete classISAbstract class

Code difference


public interface Cashier {
    void bill();

//Implementation class 1
public class CreditCart implements Cashier {
    public CreditCart(){
    public void bill(){
        System.out.println("billed by credit card");
//Implementation class 2
public class Cash implements Cashier {
    public void bill() {
        System.out.println("billed by cash");


//Abstract class
public abstract class Animal {
    String name;
    public Animal(String name) {
        this.name = name;
    public void sleep() {
    public abstract void speak();

//Implementation class 1
public class Human extends Animal {   
    public Human(String name) {
    public void speak() {
        System.out.println(name + " speack human languages");
//Implementation class 2
public class Cat extends Animal {
    public Cat(String name) {
    public void speak() {
        System.out.println(name + " speack cat's language");

Qualifier scope

# Same class Same package Sublux all
default - -
private - - -
protected -

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