Summary of the syntax rules of Java functions

Overview of functions

It is found that the continuous addition operation, in order to improve the reuse of the code, the function is packaged into an independent small program, the next time the need to perform the addition operation, you can directly call this small program, then the specific form of this encapsulation form is called a function.

Exercise: Add two integers:

1 public class FunctionDemo1{ 2 public static void main(String[] args){ 3 /* 4 int a = 4+5; 5 System.out.println("a="+a); 6 int b = 3+9; 7 System.out.println("b="+b); 11 public static int add(int a, int b){12 return a+b; 14 13}}Copy the code

For example, an automatic ticket machine in the subway, which receives notes or coins and returns tickets, has an independent function and can be understood as a function. There are unknown elements involved in the calculation (coins to put, paper money coins, how much). Have return value (return ticket)

Example 2: Mobile phone. The mobile phone has the call function and has unknown content (phone number). The keypad is (parameter) and the entered number is the actual parameter.

Function format

Modifiers return value type function name (parameter type form parameter 1, parameter type form parameter 2…)

{execute statement; Return Return value; }Copy the code

Return value type: The result of running this program. The result type is void if the function does not return a value. Function name: This is just an identifier and can be named at will. Formal argument: is a variable that stores the actual arguments passed in by the calling function. Actual parameter: The specific value passed to the formal parameter. Return value: returned to the caller.

Define a function:

1: Whether there is unknown content involved in the operation 2: Whether there is operation result (return value)

Example: Get the larger number of 2 integers.

1 public static int getMax(int x, int y) { 2 int result; 3 if (x > y) { 4 result= x; 5 } else { 6 result= y; 7 } 8 return result; 9}Copy the code

GetMax method

This method is called getMax, and the function of this method is to find the larger value of two integers. This method takes two int arguments, x and y, and returns the larger of the two numbers. GetMax (int x,int y) public static (int x,int y) public static (int x,int y) The code inside the curly braces is the method body return result; Result is the return value.

After the method definition is complete, how do YOU call a method?

Function call: To use a method, you must call it.

1 public static void main(String[] args) { 2 int max = getMax(5, 7); 3 System.out.println(max); 4}Copy the code

One: Call the getMax() method in main, where 5 and 7 are the actual arguments passed to the method. If the method has a return value, you can define a variable to receive the return value. The variable type is the same as the method’s return value type. In this case, the return value of the getMax method is received via the variable Max of type int. Full program here getMax(I, j); I and j are the actual parameters.

1 public class Demo6 { 2 public static void main(String[] args) { 3 int i=5; 4 int j=7; 5 int max = getMax(i, j); 6 system.out. println(I +" and "+j+" Max); 7 } 8 public static int getMax(int x, int y) { 9 int result; 10 if (x > y) { 11 result= x; 12 } else { 13 result= y; 14 } 15 return result; 17 16}}Copy the code

Two: in the above case, the getMax method is called and the result is assigned to the Max variable. You can also print the result of the getMax() method directly.

1 public class Demo6 { 2 public static void main(String[] args) { 3 int i=5; 4 int j=7; 6 System.out.println(getMax(I, j)); 7 } 8 public static int getMax(int x, int y) { 9 int result; 10 if (x > y) { 11 result= x; 12 } else { 13 result= y; 14 } 15 return result; 17 16}}Copy the code

Note: the main method is the entry of the program called by the virtual machine. Methods and methods cannot be nested. When the method finishes executing: The method ends when the return statement is executed, or when the curly braces at the end of the method are executed. This class contains two methods, the main method and the getMax method. The main method is called by the Java virtual machine and is written in a fixed way. The Main method can call other methods. When the getMax method is called, the value of the variable I is passed to x in the method, and the value of j is passed to Y in the method, and the statement in the getMax method is executed, and the return is executed, and the result of the operation is returned. The getMax method is complete.

Characteristics of functions

1 public static void main(String[] args) { 2 printGrade(90); 3 printGrade (59.5); 4 } 5 6 public static void printGrade(double score) { 7 char grade; 8 if (score >= 90.0) 9 system.out.println ("A"); 10 else if (score >= 80.0) 11 system.out.println ("B"); 12 else if (score >= 70.0) 13 system.out.println ("C"); 14 else if (score >= 60.0) 15 system.out.println ("D"); 16 else 17 System.out.println("E"); 18 19}Copy the code

1 public static void main(String[] args) { 2 getResult(5); 3 } 4 public static int getResult(int x){ 5 System.out.println(return x*8); 6 // An error is reported when calling this function. 7} 8 /* 9 * 7} 8 /* 9 * 7} 8 /* 9 11 times when the function is evaluated, Void void indicates that there is no specific return type. 13 * When the return type is void, the return statement in the function can be omitted. 14 */Copy the code

1 public static void main(String[] args) { 2 public static void getResult(int x) { 3 System.out.println(x * 8); 4 // Main function, inside the nested function. Error! 6 5}}Copy the code

1 /* 2 to improve code reuse 3 define a function to draw a rectangle 4 1, determine the result of the function of the data type, void 5 2, ensure that there are no unknown parameters. 6 */ 7 public static void draw(int width , int height){ 8 for(int i = 0 ; i< height ; i++){ 9 for(int j = 0 ; j < width ; j++){ 10 System.out.print("*"); 11 } 12 System.out.println(); 14 13}}Copy the code

1 /* 2 1, determine the data type of the result of the function operation, Boolean 3 2, determine that there are no unknown parameters. a,b 4 */ 5 public static boolean equlas(int a , int b){ 6 /** 7 if(a==b){ 8 return true; 9 }else{ 10 return false; 11 } 12 **/ 13 return a==b? true:false; 14 15}Copy the code

4 public static int Max (int a,int b) 5 {6 /* 7 if(a>b) 8 return a; 9 else 10 return b; 11 */ 12 13 return a>b? a:b; 14}Copy the code

1 private static void print99() { 2 for(int i = 1 ; i<= 9 ; i ++){ 3 for(int j = 1 ; j<=i ; j++){ 4 System.out.print(i+"*"+j+"="+(i*j)+" "); 5 } 6 System.out.println(); 7 } 8 } 9 private static void print99(int num) { 10 for(int i = 1 ; i<= num ; i ++){ 11 for(int j = 1 ; j<=i ; j++){ 12 System.out.print(i+"*"+j+"="+(i*j)+" "); 13 } 14 System.out.println(); 16 15}}Copy the code

void  show(int  w, double c, char b){}
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void show(int x, char y, double z){}   true       
void show(int a, double c, char b){}     false  

void show(int a, char b){}   true
void show(double c){}   true
double show(int x, char y, double z){}   true   
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