Today, I mainly learned the singleton pattern in Java and common exception error handling.

In actual development, most of the service classes will be designed as singletons.The so-called singleton mode is that the class has only one object, and the external use of the object of this class is implemented by calling a class method.

/**
   * Full Chinese singleton mode
 * @author lzq31
 *
 */
public class Service2 {
	
	private static Service2 service = new Service2();
	
	private Service2() {
		
	}
	
	public static Service2 getInstance() {
		return service;
	}
}

/**
   * Hungry Chinese singleton mode
 * @author lzq31
 *
 */
public class Service {
	
	private static Service service;
	
	private Service() {
		
	}
	
	public static Service getInstance() {
		if (null == service) {
			service = new Service();
		}
		return service;
	}
}

Common ways to handle exceptions

try catch ：

To judge whether an object is empty, usenull != str

In program method design, especially method implementation, the passed-in parameters are unknowable, so the passed-in parameters must be verified before they can be used, for example, to determine whether they are null.

public static void fun1(String str, String subStr) {
    if (null != str && null != subStr) {
        if (str.indexOf(subStr) >= 0) {
            System.out.println("Substring exists");
        } else {
                         System.out.println("There is no substring");
        }
    } else {
                 System.out.println("Illegal string");
    }

}

In the try catch structure, catch can be omitted, or multiple exceptions can be caught, but when multiple exceptions occur, the parent exception can only appear at the bottom.

In actual development, the simplest way is to use an Exception to handle directly

finally ：

It will be executed regardless of whether there is an exception or not. In particular, if you do not write catch, then finally will be executed, but the statement after the interrupt will not be executed.

public class Demo1 {
	public static void main(String[] args) {
		String str = "abc";
		str = null;
		fun1("abc", null);
	}

	public static int div(int m, int n) {
		int k = m / n;
		return k;
	}

	public static void fun1(String str, String subStr) {
		try {
			System.out.println("11111111");
			int i = 10 / 0;
			if (str.indexOf(subStr) >= 0) {
				 System.out.println("Substring exists");
			} else {
				 System.out.println("There is no substring");
			}
			System.out.println("end");
		} catch (Exception ex) {
			 System.out.println("An error occurred in the program");
			ex.printStackTrace();
			System.out.println("#"+ex.getMessage());
			
		} finally {
			 // A block of code that must be executed
			System.out.println("finally");
		}
		
		System.out.println("fun1 end");

	}
}

throws：

inMethod definitionThe following explicit declaration method is abnormal, the program that calls this method must explicitly handle the exception, and the latter can also be thrown again.

public class Demo2 {
	public static void main(String[] args) {
//		fun1();
//		fun2();
		try {
			fun3();
		} catch (Exception ex) {
			ex.printStackTrace();
		}
		
	}
	
	public static void fun1() {
		System.out.println("begin");
		int i = 0;
		try {
			i = 10 / 0;
		} catch(Exception ex) {
			ex.printStackTrace();
		}
		
		System.out.println(i);
		System.out.println("end");
	}
	
	 // Use throws to throw an exception to notify the program that called the method that you have to deal with it.
	public static void fun2() throws Exception{
		System.out.println("begin");
		int i = 0;
		i = 10 / 0;
		System.out.println(i);
		System.out.println("end");
	}
	
	public static void fun3() throws Exception {
		System.out.println("begin");
		int i = 0;
		try {
			i = 10 / 0;
		} catch (Exception ex){
			 // No processing
			 throw new Exception("System Error");
		}
		System.out.println(i);
		System.out.println("end");
	}
	
}

throw：

Create a custom exception object. When a general exception occurs, the virtual machine dynamically creates a "system exception" and throws it, which is a concept with custom throwing.

public class Demo3 {
	public static void main(String[] args) throws Exception {
		fun1(17);
		System.out.println("end");
		
	}
	
	public static void fun1(int age) throws Exception {
		if (age < 18) {
			 throw new Exception("Your age is illegal");
		} else {
			 System.out.println("You have entered the VIP room");
		}
	}
}