Java branching structure - if…else/switch

The sequential structure can only be executed sequentially and cannot make judgments and selections, so a branch structure is needed.

Java has two branching structures:

• if statement
• switch statement

if statement

An if statement contains a Boolean expression and one or more statements.

grammar

The syntax of the If statement is as follows:

 if(布尔表达式) { //如果布尔表达式为true将执行的语句}

If the Boolean expression evaluates to true, the code block in the if statement is executed. Otherwise, the code following the If statement block is executed.

 public class Test { public static void main(String args[]){ int x = 10; if( x < 20 ){ System.out.print("这是if 语句"); } } }

The results of compiling and running the above code are as follows:

这是if 语句

if...else statement

The if statement can be followed by an else statement. When the Boolean expression value of the if statement is false, the else statement block will be executed.

grammar

The usage of if…else is as follows:

 if(布尔表达式){ //如果布尔表达式的值为true }else{ //如果布尔表达式的值为false }

Example

 public class Test { public static void main(String args[]){ int x = 30; if( x < 20 ){ System.out.print("这是if 语句"); }else{ System.out.print("这是else 语句"); } } }

The results of compiling and running the above code are as follows:

这是else 语句

if...else if...else statement

The if statement can be followed by an else if...else statement, which can detect a variety of possible situations.

When using if, else if, else statements, you need to pay attention to the following points:

• An if statement has at most one else statement, and the else statement comes after all else if statements.
• An If statement can have several else if statements, and they must precede the else statement.
• Once one of the else if statements detects true, execution of the other else if and else statements will be skipped.

grammar

The syntax format of if...else is as follows:

 if(布尔表达式1){ //如果布尔表达式1的值为true执行代码}else if(布尔表达式2){ //如果布尔表达式2的值为true执行代码}else if(布尔表达式3){ //如果布尔表达式3的值为true执行代码}else { //如果以上布尔表达式都不为true执行代码}

Example

 public class Test { public static void main(String args[]){ int x = 30; if( x == 10 ){ System.out.print("Value of X is 10"); }else if( x == 20 ){ System.out.print("Value of X is 20"); }else if( x == 30 ){ System.out.print("Value of X is 30"); }else{ System.out.print("这是else 语句"); } } }

The results of compiling and running the above code are as follows:

 Value of X is 30

Nested if…else statements

It is legal to use nested if-else statements. This means you can use an if or else if statement within another if or else if statement.

grammar

The nested if…else syntax format is as follows:

 if(布尔表达式1){ ////如果布尔表达式1的值为true执行代码if(布尔表达式2){ ////如果布尔表达式2的值为true执行代码} }

You can nest else if...else just like if statements.

Example

 public class Test { public static void main(String args[]){ int x = 30; int y = 10; if( x == 30 ){ if( y == 10 ){ System.out.print("X = 30 and Y = 10"); } } } }

The results of compiling and running the above code are as follows:

 X = 30 and Y = 10

switch statement

The switch statement determines whether a variable is equal to a value in a series of values. Each value is called a branch.

grammar

The switch syntax format is as follows:

 switch(expression){ case value : //语句break; //可选case value : //语句break; //可选//你可以有任意数量的case语句default : //可选//语句}

The switch statement has the following rules:

• The variable type in the switch statement can only be byte, short, int or char. Starting from Java SE 7, switch supports the String type, and the case label must be a string constant or literal.
• A switch statement can have multiple case statements. Each case is followed by a value to be compared and a colon.
• The data type of the value in the case statement must be the same as the data type of the variable, and it can only be a constant or literal constant.
• When the value of the variable is equal to the value of the case statement, the statements following the case statement begin to execute, and the switch statement will not jump out until the break statement appears.
• The switch statement terminates when a break statement is encountered. The program jumps to the statement following the switch statement for execution. The case statement does not have to contain a break statement. If no break statement occurs, the program continues execution of the next case statement until a break statement occurs.
• A switch statement can contain a default branch, which must be the last branch of the switch statement. default is executed when no case statement is equal to the variable value. The default branch does not require a break statement.

Example

 public class Test { public static void main(String args[]){ //char grade = args[0].charAt(0); char grade = 'C'; switch(grade) { case 'A' : System.out.println("优秀"); break; case 'B' : case 'C' : System.out.println("良好"); break; case 'D' : System.out.println("及格"); case 'F' : System.out.println("你需要继续努力"); break; default : System.out.println("无效等级"); } System.out.println("你的等级是" + grade); } }

The results of compiling and running the above code are as follows:

良好你的等级是C