public class Test {

  public static void main(String args[]){
   int x = 10;

   if( x < 20 ){
     System.out.print("这是 if 语句");
   }
  }
}

这是 if 语句

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");
   }
  }
}

public class demo { 

public static void main(String[] args) { 

//设置黄文强不在 

boolean flag = false; 

System.out.println("开始"); 

if (flag){ 

System.out.println("在"); 

}else{ 

System.out.println("他不在"); 

} 

System.out.println("结束"); 

} 

}