Java study notes Day9 object-oriented and exception


  • Polymorphism means that the same method can behave in many different ways depending on the object being sent

  • The actual type of an object is determined, but there are many types of references that can point to objects

  • Conditions for the existence of polymorphism

    • have an inheritance relationship
    • Subclass override superclass method
    • Parent class reference points to child class object
  • Polymorphism is polymorphism of methods, not polymorphism of properties

public class Person {

	public void run(){
public class Student extends Person{
	public void run() {
	public void eat(){
public class Application {
	public static void main(String[] args) {

		//The actual type of an object is deterministic, the reference type that can be pointed to is indeterminate

		Student s1 = new Student();
		//The reference of the parent class points to the child class
		Person s2 = new Student();
		Object s3 = new Student();
		Student The methods that can be called are all own or inherited from the parent class
		Person The parent type, which can point to the subclass, but cannot call methods unique to the subclass
		 */;//The subclass overrides the superclass's method and executes the subclass's method;;
		//; cannot be called
Polymorphism Notes:
    1.Polymorphism is polymorphism of methods, not polymorphism of properties
    2.The parent class is related to the child class Type conversion exception ClassCastException!
    3.Existence conditions: inheritance relationship, methods need to be rewritten, parent class references point to subclass objects

    cannot be overridden:
        1.static A method belongs to a class, it does not belong to an instance constant
        3.private method

instance of and type conversion

instance of determines what type an object is

public class Application {
	public static void main(String[] args) {
		Object object = new Student();

		System.out.println(object instanceof Student);//true
		System.out.println(object instanceof Person);//true
		System.out.println(object instanceof Object);//true
		System.out.println(object instanceof Teacher);//false
		System.out.println(object instanceof String);//false
		Person person = new Student();
		System.out.println(person instanceof Student);//true
		System.out.println(person instanceof Person);//true
		System.out.println(person instanceof Object);//true
		System.out.println(person instanceof Teacher);//false
		//System.out.println(person instanceof String);//Compile error

type conversion

public class Student extends Person{
	public void go(){
public class Application {
	public static void main(String[] args) {

		//Conversion between types: parent (high) child (low)
		//When a subclass is converted to a parent class, it may lose some of its original methods
		Person student = new Student();

		//Convert the student object to the Student type, and you can call the Student type method.


1.Parent class reference points to child class object
2.Convert subclass to parent class, upcast; free conversion
3.Convert parent class to subclass, downcast; cast
4.Invocation of convenience methods

static keyword

public class Student {

	private static int age;//static variable
	private double score;//non-static variable

	public void run(){


	public static void go(){

	public static void main(String[] args) {
		Student s1 = new Student();
public class Person {

		//code block (anonymous)
		System.out.println("anonymous code block");
	static {
		//static code block
		System.out.println("static code block");

	public Person() {
		System.out.println("Construction method");

	public static void main(String[] args) {
		Person person1 = new Person();
		Person person2 = new Person();
		static code block
        anonymous code block
        Construction method
		anonymous code block
		Construction method

//static import package
import static java.lang.Math.random;
public class Test {
	public static void main(String[] args) {

abstract class

  • The abstract modifier can be used to modify a method or a class. If a method is modified, then the method is an abstract method; if a class is modified, then the class is an abstract class
  • Abstract classes can have no abstract methods, but classes with abstract methods must be declared as abstract classes
  • Abstract class, cannot use the new keyword to create objects, it is used for subclasses to inherit
  • Abstract method, only method declaration, no method implementation, it is used for subclasses to implement
  • If a subclass inherits an abstract class, it must implement the abstract methods that the abstract class does not implement, otherwise the subclass must also be declared as an abstract class
//abstract abstract class class can only inherit single interface can inherit multiple
public abstract class Action {
	//Abstract method, only the name of the method, no implementation of the method
	public abstract void doSomething();

1.The abstract class cannot be new, it can only be realized by subclasses; constraints
2.Ordinary methods can be written in abstract classes
3.Abstract method must be in abstract class
 The essence is restraint
 Constructor exists in abstract class
//All methods of an abstract class, inheriting its subclasses, must implement his methods
public class A extends Action{
	public void doSomething() {


Ordinary classes: only concrete implementations

Abstract class: both concrete implementation and specification (abstract method)

Interface: only specs! Constraint and Implementation Separation: Interface-Oriented Programming

An interface is a specification that defines a set of rules that embody the real-world "if you are... you must be able to..." mentality. Example: If you are a bird, you must be able to fly.

The essence of an interface is a contract, just like the laws of our world. After the formulation is completed, everyone will abide by

The essence of object-oriented is the abstraction of objects, and the interface that best reflects this is the interface

//An interface needs to have an implementation class
public interface UserService {

	public static final int age = 99;

	//All methods defined in the interface are actually abstract by default public abstract
	 void add(String name);
	 void delete(String name);
	 void update(String name);
	 void query(String name);
public interface TimeService {
	void timer();
//class implements interface
//A class that implements an interface needs to override the methods in the interface

//Implementing Multiple Inheritance Using Interfaces
public class UserServiceImpl implements UserService,TimeService{
	public void add(String name) {


	public void delete(String name) {


	public void update(String name) {


	public void query(String name) {


	public void timer() {

    2.Define some methods for different people to implement
    3.public abstract
    4.public static final
    5.The interface cannot be instantiated, there is no constructor in the interface
    6.implements Can implement multiple interfaces
    7.You must override the methods in the interface

inner class

An inner class is to define another class inside a class. For example, if a class B is defined in class A, then class B is called an inner class relative to class A, and class A is an external class relative to class B.

  • member inner class
  • static inner class
  • local inner class
  • anonymous inner class
public class Outer {

	private int id = 123;
	public void out(){
		System.out.println("This is the method of the outer class");
	 public class inner{
		public void in(){
			System.out.println("This is the method of the inner class");

		 //Get private properties of outer class
		 public void getId(){

public class Application {
	public static void main(String[] args) {
		Outer outer = new Outer();

		//Instantiate inner class from outer class
		Outer.inner inner = inner();


In actual work, the situation encountered cannot be perfect. For example, for a module you wrote, the user input may not meet your requirements, your program wants to open a certain file, the file may not exist or the file format is wrong, you want to read the data of the database, the database may be empty . Our program is running and the memory or hard drive may be full.

During the running process of the software program, it is very likely to encounter these abnormal problems just mentioned. We call it abnormal. English is: Exception, which means exception. These, exceptions, or exceptions, how can the programs we write make reasonable processing, so as not to cause the program to crash.

Exceptions refer to various situations that occur unexpectedly during program operation, such as: file cannot be found, network connection failure, illegal parameters, etc.

An exception occurs during program execution, and it affects the normal program execution flow.

There are three types of exceptions in Java:

  • Checked Exceptions: The most representative checked exceptions are those caused by user errors or problems, which the programmer cannot foresee. For example, when opening a file that does not exist, an exception occurs. These exceptions cannot be simply ignored at compile time.
  • Runtime Exceptions: Runtime exceptions are exceptions that may be avoided by the programmer. In contrast to checked exceptions, runtime exceptions can be ignored at compile time.
  • Errors: Errors are not exceptions, but problems outside the programmer's control. Errors are usually ignored in code. For example, when the stack overflows, an error occurs and they cannot be checked by the compilation

Java treats exceptions as objects and defines a base class java.lang.Throwable as the superclass of all exceptions.

Many exception classes have been defined in the Java API, and these exception classes are divided into two categories, Error and Exception.


Error class objects are generated and thrown by the Java virtual machine, most errors have nothing to do with the actions performed by the code writer

An OutOfMemoryError occurs when the JVM no longer has the necessary memory resources to continue the operation. When these exceptions occur, the Java virtual machine generally chooses to terminate the thread

It also occurs when the virtual machine tries to execute the application, such as class definition error (NoClassDefFoundError), connection error (LinkageError). These errors are untraceable because they are outside the control and processing power of the application, and most are not allowed to occur while the program is running.


There is an important subclass RuntimeException (runtime exception) in the Exception branch

  • ArrayIndexOutOfBoundsException (array index out of bounds)
  • NullPointerException (null pointer exception)
  • ArithmeticException
  • MissingResourceException (missing resource)
  • ClassNotFoundException (class not found), these exceptions are unchecked exceptions, and the program can choose to capture and handle them, or not to handle them.

These exceptions are generally caused by program logic errors, and the program should avoid the occurrence of such exceptions as much as possible from a logical point of view;

The difference between Error and Exception: Error is usually a catastrophic fatal error that cannot be controlled and handled by the program. When these exceptions occur, the Java virtual machine generally chooses to terminate the thread; Exception can usually be handled by the program, and These exceptions should be handled as much as possible in the program.

exception handling mechanism

  • Throw an exception
  • catch exception
  • Five keywords for exception handling
    • try,catch,finally,throw,throws
public class Test {
	public static void main(String[] args) {

		new Test().test(1,0);
	//Suppose this method cannot handle this exception. method throws exception

	public void test(int a,int b) throws ArithmeticException{
		if (b==0) {//throw throws
			throw new ArithmeticException();//Actively throw exceptions, generally used in methods


int a = 1;
		int b = 0;

		//Suppose you want to catch multiple exceptions: small to large
		//Shortcut: CTRL alt + t

		try {//try Monitoring area
			System.out.println(a / b);
		}catch (ArithmeticException e){//catch(the type of exception you want to catch) catch the exception
			System.out.println("An exception occurred in the program");
		}finally {//handle the aftermath

		//can not finally,


custom exception

Using Java's built-in exception classes can describe most of the exceptions that occur during programming. In addition, users can also customize exceptions. User-defined exceptions, just inherit the Exception class.

Using a custom exception class in a program can be roughly divided into the following steps

  • Create custom exception class
  • Throws an exception object through the throw keyword in a method
  • If the exception is handled in the method that currently throws the exception, you can use the try-catch statement to capture and handle it; otherwise, specify the exception to be thrown to the caller of the method through the throws keyword at the declaration of the method, and proceed to the next step.
  • Catch and handle exceptions in the caller of the exception method.
//custom exception class
public class MyException extends Exception{

	//Pass a number greater than 10, throw an exception
	private int detail;

	public MyException(int a) {

	//toString: abnormal print information
	public String toString() {
		return "MyException{" +
				"detail=" + detail +
public class Test1 {

	//There may be exception methods
	static void test1(int a ) throws MyException {
		System.out.println("The parameter passed is"+a);

		if (a>10){
			throw new MyException(a);

	public static void main(String[] args) {
		try {
		} catch (MyException e) {


  • When dealing with running exceptions, use logic to reasonably avoid and assist with try-catch processing
  • After multiple catches, you can add a catch (Exception) to handle exceptions that may be missed
  • For uncertain code, try-catch can also be added to handle potential exceptions
  • Try to handle exceptions as much as possible, don't simply call printStackTrace() to print the output
  • How to deal with exceptions depends on different business requirements and exception types.
  • Try to add finally block to release occupied resources

Tags: Java Programming

Posted by DJ Judas on Fri, 03 Jun 2022 07:53:16 +0530