Java练习一

Java编码示例

各种java程序来说明多种概念。

• 输出hello world
  /*Hello,world
  */
  public class HelloWorld{
  public static void main(String[] args){
  System.out.println("Hello,world");
  }
  }

• 如何在同一个类中调用方法的示例

  /*CallingMethodsInSameClass.java
  *说明如何在同一个类中调用静态方法
  */
  public class CallingMethodsInSampleClass{
  public static void main(String[] args){
  printOne();
  printOne();
  printTwo();
  }

    public static void printOne(){
        System.out.println("Hello world");
    }
    
    public static void printTwo(){
        printOne();
    }
  

  }

• 使用for循环计算阶乘的简单示例。使用内置的int数据类型只有显示13！
  public class Factorial{
  public static void main(String[] args){
  final int NUM_FACTORS = 100;
  for(int i < 0; i < NUM_FACTORS; i ++)
  {
  System.out.println(i + "! is" + factorial(i));
  }
  }

      public static int factorial(int n){
          int result = 1;
          for(int i = 2;i < =n ;i++){
              result *= i;
          }
          return result;
      }
  }
  

• 增强的循环
  public class EnhancedFor{
  public static void main(String[] args){
  int[] list = {1,2,3,4,5,6,7,8,9};
  int sum = sumListEnhanced(list);
  System.out.println("Sum of elements in list: " + sum);

  		System.out.println("Original List");
  		printList(list);
  		System.out.println("Calling addOne");
  		addOne(list);
  		System.out.println("List after call to addOne");
  		printList(list);
  		System.out.println("Calling addOneError");
  		addOneError(list);
  		System.out.println("List after call to addOneErrro.Noete elements of list did not change");
  		printList(list);
  	}
  	//使用加强的循环语句,进行求和
  	public static int sumListEnhanced(int[] list){
  		int total = 0;
  		for(int var:list){
  			total += var;
  		}
  		return total;
  	}
  
  	//使用传统的循环,进行求和
  	public static int sumListold(int[] list){
  		int total = 0;
  		for(int i = 0; i < list.length; i++){
  			total += list[i];
  			System.out.println(list[i]);
  		}
  		return total;
  	}
  
  	//下面的代码好像是给每个元素加1,实际上并没有
  	public static void addOneError(int[] list){
  
  		for(int val:list){
  			val += 1;
  		}
  	}
  
  	//给每个元素加1
  	public static void addOne(int[] list){
  		for(int i = 0; i < list.length; i++){
  			list[i] += 1;
  		}
  	}
  
  	//打印每个数组
  	public static void printList(int[] list){
  		System.out.println("index, value");
  		for(int i = 0; i<list.length;i++){
  			System.out.println(i + ", "+ list[i]);
  		}
  	}
  }
  

• 显示值参数行为的例子。在Java中，所有参数都是按照值进行传递
  public class PrimitiveParameters{
  public static void main(String[] args){
  go();
  }

  	public static void go(){
  		int x = 3;
  		int y = 2;
  		System.out.println("In method go. x:" + x +" y: "+y);
  		falseSwap(x,y);
  		System.out.println("In method go. x:" + x +" y: "+y);
  		moreParameters(x,y);
  		System.out.println("In method go. x:" + x+ "  y: "+y);
  	}
  
  	public static void falseSwap(int x,int y){
  		System.out.println("In method go. x:" + x +" y: "+y);
  		int temp = x;
  		x = y;
  		y = temp;
  		System.out.println("in method falseSwap. x:"+x + "y:"+y);
  	}
  
  	public static void moreParameters(int a, int b){
  
  		System.out.println("in method falseSwap. a:"+a + " b:"+b);
  		a = a*b;
  		b = 12;
  		System.out.println("in method falseSwap. a:"+a + " b:"+b);
  		falseSwap(b,a);
  		System.out.println("in method falseSwap. a:"+a + " b:"+b);
  	}
  }
  
  //下面是执行结果
  In method go. x:3 y: 2   //初始化x=3,y=2
  In method go. x:3 y: 2   //执行falseSwap函数，以在函数内成功调整两数值
  in method falseSwap. x:2y:3
  In method go. x:3 y: 2 //执行完falseSwap函数并没有交换两值
  in method falseSwap. a:3 b:2 //执行moreParameters
  in method falseSwap. a:6 b:12
  In method go. x:12 y: 6
  in method falseSwap. x:6y:12
  in method falseSwap. a:6 b:12
  In method go. x:3  y: 2 //对两个实际值并没有改变
  

• 一些String操作的简短例子
  public class StringExample{
  public static void main(String[] args){
  String s1 = "Computer Science";
  int x = 307;
  String s2 = s1 + " " +x;
  String s3 = s2.substring(10,17);
  String s4 = "is fun";
  String s5 =s2 + s4;

  		System.out.println("s1: "+s1);
  		System.out.println("s2: "+s2);
  		System.out.println("s3: "+s3);
  		System.out.println("s4: "+s4);
  		System.out.println("s5: "+s5);
  
  		x = 3;
  		int y = 5;
  		String s6 = x + y + "total";
  		String s7 = "total" + x + y;
  		String s8 = ""+x+y + "total";
  		System.out.println("s6: "+s6);
  		System.out.println("s7: "+s7);
  		System.out.println("s8: "+s8);
  
  	}
  }
  
  //执行结果
  //Java编译器是从左到右处理表达式的，s6第一个表达式x+y的话就是Java编译器理解为运算符，而第二和第三则理解为连接符
  s1: Computer Science
  s2: Computer Science 307
  s3: cience 
  s4: is fun
  s5: Computer Science 307is fun
  s6: 8total
  s7: total 35
  s8: 35 total
  

• 一个程序，包含各种Java语法的示例，它将10进制转化2进制
  public class BinaryConverter{
  public static void main(String[] args){
  for(int i = -5;i<32;i++){
  System.out.println(i+": "+ toBinary(i));
  System.out.println(i);
  System.out.println(i+": "+Integer.toBinaryString(i));
  }
  }

  	//在base2中返回一个base10 num的字符串
  	public static String toBinary(int base10Num){
  		boolean isNeg = base10Num < 0; // 优先级，先计算<，base10Num小于0为真
  		base10Num = Math.abs(base10Num); //返回绝对值Math.abs()
  		String result = ""; 
  
  		while(base10Num > 1){
              //以base10Num等于11为例子，result = 11%2+ ' ' = '1'
              //base10Num/2 = 5
              //result = 5%2 + '1'='11'
              //base10Num/2=1
              //result = 1%2 +'11'='011'
              //base10Num = 1 
  			result = (base10Num %2) + result; 
  			base10Num /= 2;
  		}
          //断言 asser condition:expr;检查值是否等于0或1
  		assert base10Num == 0 || base10Num == 1:"value is not <= 1: "+base10Num;
  		
          //还是以11为例，经过上面的循环,result='011'，base10Num=1
          //最终result=1011
  		result = base10Num + result;
          
          //断言
  		assert all0sAndls(result);
  		
          //如果是false，不执行。正数不执行，负数执行
  		if( isNeg )
  			result = "-" + result;
  
  		return result;
  	}
  
  	
      public static boolean all0sAndls(String val){
  		//判断result是否有结果
          assert val != null: "Failed precondition all0sAndls.parameter cannot be null";
  		boolean all = true;
  		int i = 0;
  		char c;
          //判断每个位置是否是0或1
  		while(all && i < val.length()){
              //charAt() 方法用于返回指定索引处的字符。
  			c = val.charAt(i);
  			all = c == '0' || c == '1';
  			i++;
  		}
  		return all;
  	}
  }