One ,true and false

        java A number is not allowed as a Boolean value , But in the C/C++ Is allowed in , as follows :
void example15(){ if(1){// stay C In language ,1 It represents the truth ,0 It's a fake ,C++ It's the same thing printf("true"); }else{
printf("false"); } }
        java Must use a conditional expression to convert it to a boolean type value . of course , Boolean values can also be used directly , as follows :
public void example1(){ if(true){// Direct use of true and false values System.out.println("true"); } int t
= 3; if(t>3){// Conditional expression System.out.println("true"); } }
Two ,if-else

if-else Is the most basic form of control program flow , as follows :
public void example2(){ /** * if-else */ int score = 90; if(score>0){
System.out.println(" You passed !"); }else {//else Statements can be omitted System.out.println(" You failed !");
} /** * if-else-if-else */ int week = 2; if(week == 1){
System.out.println(" Today is Monday "); }else if(week == 2){
System.out.println(" Today is Tuesday "); }else { //...... } }
Three , iteration

1,while
public void example3(){ int i = 0; int j = 0; while (i<5){ j += i; i++; }
System.out.println(j);//0+1+2+3+4=10 }
2,do-while
@Test public void example4(){ int i = 0; int j = 0; do{ j += i; i++; }while
(i<5); System.out.println(j);//0+1+2+3+4=10, It follows while The difference is that it is executed once }
3,for
/** * from 1 Add 100 */ @Test public void example5() { int j = 0; for (int i = 1; i
<= 100; i++) { j += i; } System.out.println(j);//5050 }
4,foreach
public void example6(){ /** * foreach array */ int[] nums = {10,20,30,40,50}; int
result = 0; for(int i:nums){ result += i; } System.out.println(result);//150
/** * foreach list aggregate */ List<String> strings = new ArrayList<>();
strings.add(" Zhang San "); strings.add(" Li Si "); strings.add(" WangTwo "); String str =
" There are three people in this collection , namely :"; for(String s:strings){ str += s + ","; } str =
str.substring(0,str.length()-1);
System.out.println(str);// There are three people in this collection , namely : Zhang San , Li Si , WangTwo /** * foreach map aggregate */
Map<String,String> map = new HashMap<>(); map.put(" Zhang San ","20");
map.put(" Li Si ","25"); map.put(" Wang Wu ","19"); for(String s:map.keySet()){
System.out.println(s + " What's your age :" + map.get(s) + " year !"); } /** * What's the age of Li Si :25 year ! *
What's the age of Zhang San :20 year ! * What is Wang Wu's age :19 year ! */ }
5,return

       
return Keywords serve two purposes : One is to specify what value a method returns , The second is to cause the current method to exit . If you are returning void None of the methods return sentence , Then at the end of the method, there is an implicit return.
@Test public void example9() { Boolean b = this.back1(10, 5, 20);
System.out.println(b); Boolean b1 = this.back1(25, 5, 20);
System.out.println(b1); } // Judge whether it is between two numbers private Boolean back1(Integer num,
Integer begin, Integer end) { if (num >= begin && num <= end) { return
Boolean.TRUE; } return Boolean.FALSE; }
6,break and continue

        break Used to force an exit loop , The remaining statements in the loop are not executed , and continue Is to abort the current loop , Continue to the next cycle .
@Test public void example10(){ System.out.println(useBreak(10));
System.out.println(useContinue(10)); } private int useBreak(int num){ int
result = 0; for(int i=1;i<num;i++){ if(i%5 == 0){ break; } result += i; }
return result;//1+2+3+4=10 } private int useContinue(int num){ int result = 0;
for(int i=1;i<num;i++){ if(i%5 == 0){ continue; } result += i; } return
result;//1+2+3+4+6+7+8+9=40 }
7,java Label and goto

       
goto It's a jump at the source level , This has led to a bad reputation , If a program always jumps from one place to another , This makes it difficult to identify the control flow of the program . in fact , The real problem is not the use goto, It's about goto The abuse of , In a few cases ,goto It is the best way to control the process ,goto Can be used to jump out of deep nested loops . although goto still java A reserved word in , But it's not used in language .

       
java Using tags ,break and continue Keyword jump operation , Tagged break It breaks the loop and jumps out of the loop that the tag refers to , Tagged continue It will reach the location of the label , And re-enter the loop immediately after that tag .
@Test public void example11(){ // Three groups , Each group 5 Students int[][] arr =
{{65,80,92,85,55},{58,65,76,52,66},{67,63,52,85,76}}; int result = 0;
out:for(int i=0;i<arr.length;i++){ for(int j=0;j<arr[i].length;j++){
if(i==0){// End the first cycle of the outer cycle , Go to the next cycle continue out; }
if(i==2){// When the outer cycle enters the third time , End the outer cycle directly break out; } result =
result>arr[i][j]?result:arr[i][j]; } } System.out.println(result);//76 }
8,switch

         each case It's a tag, When matching to tag The others are ignored tag The code body is then executed sequentially , Until the encounter break perhaps }
@Test public void example12(){ int i=9; switch (i) { default:
System.out.println("default"); case 0: System.out.println("zero"); case 1:
System.out.println("one"); case 2: System.out.println("two"); } /* * default *
zero * one * two analysis : each case It's a tag, When matching to tag The others are ignored tag The code body is then executed sequentially , Until the encounter break perhaps } */ }
 

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