Methods, functions

1、 Concept

Concept ： A piece of code that implements a specific function , Can be used repeatedly .

2、 Definition

• Modifier ：public 、static It's called a modifier
• return type ：⽤ To illustrate the ⽅ Type of operation result . If you return other types , Compilation can go wrong
• ⽅ Legal name ： It acts as a key ⽤ Time quote ⽤⽅ Identification of law
• Parameter list ： stay ⽅ The law has been adjusted ⽤ when ⽤ Accept external communication ⼊ The variables of are called formal parameters, which are called formal parameters for short ,⽅ The number of formal parameters of method can be 0 A to
  Multiple , Before each parameter ⾯ The data type to declare parameters is called parameter type ; Each parameter should ⽤ Comma separated . It's fine too ⼀ There are no parameters
• ⽅ Dharma body ： It is ⼀ A statement block , Of board ⾏ Specific function operation . For those with return value type ⽅ Law ,⽅ At the end of the Dharma body ⼀ The first sentence is
  return keyword , Its work ⽤ It's a ⽅ The enforcement of law ⾏（ operation ） The result goes back to ⽅ External law
• return expression ：return Is the key word , do ⽤ Yes, it will return after ⾯ The result of the expression is ⽅ The return value of the method . Pay attention to the expression
  type , Must be with ⽅ Declared in the header “ Return type ” Match

 grammar ：
public static void  Method name (){
    
// Method theme 
}
    
 Methods are defined inside the class , On main Method juxtaposition 

demonstration ：

    public static void main(String[] args) {
    
        xing();// call xing Method 
        System.out.println(" abed, I see a silver light ");
        xing();
        System.out.println(" The frost on the ground ");
        xing();
        System.out.println(" look at the bright moon ");
        xing();
        System.out.println(" Bow your head and think of your hometown ");
        xing();
    }

    public static void xing() {
    
        System.out.println("************");
    }

3、 call

1. classification

   Ginseng 、 No arguments 、 There is a return value 、 No return value

   //⽆ ginseng ⽆ The return value of ⽅ Law 
   public static void method1(){
         
   System.out.println(" I am ⾃ entertainment ⾃ Happy ");
   }
   // Ginseng ⽆ The return value of ⽅ Law 
   public static void method2(int a){
         
   // Calculation a flat ⽅
   System.out.println(a*a);
   }
   //⽆ Parameter with return value ⽅ Law 
   public static long method3(){
         
   // Get the current time 
   long time = System.currentTimeMillis();
   return time;
   }
   // With parameter and return value 
   public static int method4(int a,int b){
         
   // return a,b The median value is ⼤ the 
   return a>b?a:b;
   }
   

2. Call mode

   • Call alone Methods are independent of other code , Perform functions independently
     • Multi use inspection and maintenance methods
   • Output call Only output the results , This call means nothing to us
   • Assignment call There is a return value result , You can do the next step

public static void main(String[] args) {
    
    // Assignment tone ⽤
    int c = multipy(14, 15);
    System.out.println(c);
    // Output modulation ⽤
    print();
    // Adjust separately ⽤
    clear();
}
// Definition ⽅ Law , Calculate the product of two numbers 
public static int multipy(int a, int b){
    
    int count = a * b;
    return count;
}
// Print statement function 
public static void print(){
    
    System.out.println(" wow , Beautiful ⼥....");
}
// System garbage cleaning function 
public static void clear(){
    
    for (int i = 0; i < 1000; i++) {
    
        System.out.println(" System cleaning ....");
    }
}

4、 benefits

Reduce code redundancy 、 Improve reusability , Readability , Maintainability , Facilitate division of labor and cooperation

5、 heavy load

1. Definition

   There are multiple methods with the same name in a class , Methods with different parameter lists are called method overloading

2. parameter list

   • The number of parameters is different
   • Different parameter types
   • The order of the parameters is different

public static void method(int i,double d){
    
}
// And the ⼀ individual ⽅ The number of parameters of method is different 
public static void method(int i){
    
}
// And the ⼀ individual ⽅ The types of parameters of method are different 
public static void method(String str,float f){
    
}
// And the ⼀ individual ⽅ The order of the parameters of the method is different 
public static void method(double d,int i){
    
}

6、 Methods call each other memory graph

1. Definition of stack
   • Run methods and store local variables
   • A stack is a tube with an opening at one end , Similar to badminton tube

2. Execute the process

   When JVM When executing a method , Will put this method on the stack to run . When this method calls other methods ,JVM The new method will also be placed on the existing method in the stack for execution , When the method is finished , The method will be JVM Remove... From the stack , It's called a bomb stack .

7、 Method parameters

Most of the time , Data interaction is required between the method and the caller ; The caller must provide the necessary data , In order to make the method complete the corresponding function .

Parameter function ： The parameters of the method can make the code function more flexible 、 Higher universality , Easy to modify and maintain .

Formal parameter and actual parameter

A single parameter uses

demand ： On multiple calls xing When the method is used , You can print minus signs for different times

    public static void main(String[] args) {
    
        xing(5);
        System.out.println(" abed, I see a silver light ");
        xing(10);
        System.out.println(" The frost on the ground ");
        xing(2);
        System.out.println(" look at the bright moon ");
        xing(15);
        System.out.println(" Bow your head and think of your hometown ");
        xing(5);

    }

    public static void xing(int count) {
    
        for (int i = 0; i <= count; i++) {
    
            System.out.print("*");
        }
        System.out.println();
    }

Multiple parameters use

demand ： On multiple calls xing When the method is used , You can print symbols for different times

    public static void main(String[] args) {
    
        xing(5,"*");
        System.out.println(" abed, I see a silver light ");
        xing(10,"+");
        System.out.println(" The frost on the ground ");
        xing(2,"&");
        System.out.println(" look at the bright moon ");
        xing(15,"%");
        System.out.println(" Bow your head and think of your hometown ");
        xing(5,"@");

    }

    public static void xing(int count,String hao) {
    
        for (int i = 0; i <= count; i++) {
    
            System.out.print(hao);
        }
        System.out.println();
    }

Return value and return type

 Concept ： The returned result after the method is executed .
 After method execution , In some cases, there is no need to return results ; In other cases, the result must be returned .

Example ：
demand ： Define methods , Calculate the sum of two integers , And return the result , stay main Print in .

public class work2 {
    
    public static void main(String[] args) {
    
        double add=countNum(51.1, 2);
        System.out.println(add);
    }

    public static double countNum(double num1, double num2) {
    
        double sum = num1 + num2;
        return sum;
    }

Definition countNum Method has two parameters , Let them add up , Use return Return the sum . By calling countNum Assign a value to double num1, double num2.

 Each method has a return value ,if Statement and so on, each branch has a return value .

return In two ways ：
     Used in methods that have return value types ：
        return value;// Indicates the end of the current method , With a return value , Return to method call .
     The application has no return value type （void） The methods of ：
        return;// Indicates the end of the current method , Return directly to the method call .

Three keywords

keyword	effect
continue	End this cycle
break	End the cycle , The code after the loop continues to execute
return	End the whole method

8、 Numbers as parameters 、 Return value

As a parameter

Arrays are passed as method parameters , The parameter passed is the memory address of the array

public static void main(String[] args) {
    
    int[] arr = {
     1, 3, 5, 7, 9 };
    // transfer ⽤⽅ Law , Pass array 
    printArray(arr);
}
/*  establish ⽅ Law ,⽅ Method to receive parameters of array type   Into the ⾏ Traversal of array  */
public static void printArray(int[] arr) {
    
    for (int i = 0; i < arr.length; i++) {
    
        System.out.println(arr[i]);
    }
}

As return value

Array as ⽅ The return value of the method , It returns the memory address of the array

public static void main(String[] args) {
    
    // transfer ⽤⽅ Law , Receive the return value of the array 
    // Received memory address of array 
    int[] arr = getArray();
    for (int i = 0; i < arr.length; i++) {
    
        System.out.println(arr[i]);
    }
}
/* establish ⽅ Law , The return value is an array type  return Return the address of the array  */
public static int[] getArray() {
    
    int[] arr = {
     1, 3, 5, 7, 9 };
    // Return the address of the array , Return to the key ⽤ person 
    return arr;
}

⽅ When the parameters of the method are basic types , What's passed is the data value . ⽅ The parameters of the method are ⽤ Type , What is passed is the address value .

9、 Variable parameters

1. Definition

   Method declaration in , Represents multiple parameters of the same type

2. matters needing attention

   In the actual call , Pass in multiple arguments within a method , Virtual Opportunities automatically help us fill in variable parameters , Form an array

3. Example

   public void method(String... strs){
         
       for (int i = 0; i < strs.length; i++) {
         
           System.out.println(strs[i]);
       }
   }
   public static void main(String[] args) {
         
       Class class1 = new Class();
       class1.method("⼩ bright ","⼩ red ","⼩ when ");
   }
   

10、 recursive

recursive

1. Definition

   Recursion is dealing with regular problems , Call your own method again inside the method

2. Use scenarios

   • When a problem can be split into multiple problems , The treatment methods of size problems are consistent , Method calls itself
   • Routine problems that can be handled with loops , Can use recursive processing

3. Use

   Exit must be set , Let each method return correctly , Cannot call infinitely
   

Factorial recursion

demand ： Calculation 5 The factorial ：5！=54321;

    public static void main(String[] args) {
    
        System.out.println(countNum(5));
    }

    public static int countNum(int n) {
    
        int num = 1;
        for (int i = 1; i <= n; i++) {
    
            num = num * i;
        }
        return num;
    }

Recursive computation

    public static void main(String[] args) {
    
        System.out.println(Five(5));
    }

    //5!=5*4! n yes 5,Four(n - 1) Return to Four=4
    public static int Five(int n) {
    
        return n * Four(n - 1); //5*4*3*2*1 n=5
    }

    public static int Four(int n) {
    
        return n * Three(n - 1);    //4*3*2*1 n=4
    }

    public static int Three(int n) {
    
        return n * Two(n - 1);  //3*2*1 n=3
    }

    public static int Two(int n) {
    
        return n * one(n - 1);  //2*1 n=2
    }

    public static int one(int n) {
    
        return 1;    //1 n=1
    }

In fact, the factorial method has been repeated in writing , We extract the repeated part , The optimizer .

    public static void main(String[] args) {
    
        System.out.println(Factorial(5));
    }
    // Special factorial method 
    public static int Factorial(int n) {
    
        // Set the exit of program recursion , Otherwise, a memory overflow error will occur 
        if (n == 1) {
    
            return 1;
        }
        //5*4！, then Factorial(n - 1) Will call itself , So it becomes 4*3！, Always recursive 
        return n * Factorial(n - 1);
    }

Factorial It's a special way to calculate factorials , Call itself automatically , Repeated recursion , But we need to consider the export of the program , That is to say n=1 Back when 1, End procedure , Can't make n=-1 Calculate it , Otherwise, there will be a memory overflow error .
ic void main(String[] args) {
System.out.println(Factorial(5));
}
// Special factorial method
public static int Factorial(int n) {
// Set the exit of program recursion , Otherwise, a memory overflow error will occur
if (n == 1) {
return 1;
}
//54！, then Factorial(n - 1) Will call itself , So it becomes 43！, Always recursive
return n * Factorial(n - 1);
}

Factorial It's a special way to calculate factorials , Call itself automatically , Repeated recursion , But we need to consider the export of the program , That is to say n=1 Back when 1, End procedure , Can't make n=-1 Calculate it , Otherwise, there will be a memory overflow error .