Java 如何循环遍历对象的ArrayList并使其打印所有内容？

几天前我问了一个问题，现在我有一个更具体的问题，因为我在我的程序上做了更多的工作，并添加了一些东西。基本上，我从一个空的ArrayList开始，它应该保存通过控制台添加的生日。我记下的这部分+它打印我添加的生日。但是如果我想添加另一个，它会打印我再次添加的第一个？我怎样才能让它打印出迄今为止我添加的所有生日？我将显示到目前为止的代码

生日班

public class Birthday {
    private String bdayKid;
    private int age;
    private boolean gift;
    
    public Birthday(String bdayKid, int age, boolean gift) {
        this.bdayKid = bdayKid;
        this.age = age;
        this.gift = gift;
    }
    
    //overridden toString() method
    public String toString() { 
        return this.bdayKid + " turns " + this.age + "! They are" + 
            (this.gift ? "" : "not ") + " getting a gift.";
    }
}

public class MainClass{
   public static void main(String []args) {
      ArrayList<Birthday> bdays = getBirthdays();
      printBirthdays(bdays);
   }

   //This method will return a list of birthdays
   public ArrayList<Birthday> getBirthdays() { 
      ArrayList<Birthday> bdays = new ArrayList<Birthday>();
      Scanner scan = new Scanner(System.in);
      
      //create a birthday
      Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
      //add the birthday to arraylist of birthdays
      bdays.add(bday);

      
      return bdays;
   }

   //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         System.out.println(bday);
      }
   }
}

public class MainClass{
   public static void main(String []args) {
      // Instantiate your one and only bdays array
      ArrayList<Birthday> bdays = new ArrayList<Birthday>();
      getBirthdays(bdays);
      printBirthdays(bdays);
   }

   //This method will return a list of birthdays
   public void getBirthdays(ArrayList<Birthday> bdays) { 
      Scanner scan = new Scanner(System.in);
      
      //create a birthday
      Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
      //add the birthday to arraylist of birthdays
      bdays.add(bday);

      // Good practice to close a scanner
      scan.close();
   }

   //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         System.out.println(bday);
      }
   }
}

主类

public class Birthday {
    private String bdayKid;
    private int age;
    private boolean gift;
    
    public Birthday(String bdayKid, int age, boolean gift) {
        this.bdayKid = bdayKid;
        this.age = age;
        this.gift = gift;
    }
    
    //overridden toString() method
    public String toString() { 
        return this.bdayKid + " turns " + this.age + "! They are" + 
            (this.gift ? "" : "not ") + " getting a gift.";
    }
}

public class MainClass{
   public static void main(String []args) {
      ArrayList<Birthday> bdays = getBirthdays();
      printBirthdays(bdays);
   }

   //This method will return a list of birthdays
   public ArrayList<Birthday> getBirthdays() { 
      ArrayList<Birthday> bdays = new ArrayList<Birthday>();
      Scanner scan = new Scanner(System.in);
      
      //create a birthday
      Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
      //add the birthday to arraylist of birthdays
      bdays.add(bday);

      
      return bdays;
   }

   //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         System.out.println(bday);
      }
   }
}

public class MainClass{
   public static void main(String []args) {
      // Instantiate your one and only bdays array
      ArrayList<Birthday> bdays = new ArrayList<Birthday>();
      getBirthdays(bdays);
      printBirthdays(bdays);
   }

   //This method will return a list of birthdays
   public void getBirthdays(ArrayList<Birthday> bdays) { 
      Scanner scan = new Scanner(System.in);
      
      //create a birthday
      Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
      //add the birthday to arraylist of birthdays
      bdays.add(bday);

      // Good practice to close a scanner
      scan.close();
   }

   //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         System.out.println(bday);
      }
   }
}

---

是否需要在for each循环中添加if语句？感谢您的帮助

在

生日

类中添加字段：

private boolean printed = false;

然后在打印功能上：

 //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         if(!bday.isPrinted())
         {
            System.out.println(bday);
            bday.setPrinted(true);
         }
      }
   }

//此方法将打印生日列表
公共无效打印生日（ArrayList B天）{
//重复每个生日并打印结果
生日（B日：B日）{
如果（！bday.isPrinted（））
{
系统输出打印项次（b天）；
b日期设置打印（真）；
}
}
}

在

生日

类中添加字段：

private boolean printed = false;

然后在打印功能上：

 //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         if(!bday.isPrinted())
         {
            System.out.println(bday);
            bday.setPrinted(true);
         }
      }
   }

//此方法将打印生日列表
公共无效打印生日（ArrayList B天）{
//重复每个生日并打印结果
生日（B日：B日）{
如果（！bday.isPrinted（））
{
系统输出打印项次（b天）；
b日期设置打印（真）；
}
}
}

您在理解Java方面遇到了严重的问题。首先，你需要理解一个概念，一个变量的范围

如果在函数中创建变量，则变量的作用域仅在函数中

您有标识符为

bdays

的

ArrayList

的不同实例。在

void main

函数中定义的一个实例，以及每次调用

getBirthdays（）

方法时创建的其他实例。在switch语句中，当调用

getBirthdays（）

时，函数将形成并返回一个新的

ArrayList

，而原始的

ArrayList bdays

保持不变。当您调用

printBirthdays（bdays）

时，它将参数作为您在主方法的第一行中创建的数组，而不是

getBirthdays（）

方法返回的新

ArrayList

。因此，它不会打印新的

Brithday

。因此，您可以按如下方式更改代码：

主类

public class Birthday {
    private String bdayKid;
    private int age;
    private boolean gift;
    
    public Birthday(String bdayKid, int age, boolean gift) {
        this.bdayKid = bdayKid;
        this.age = age;
        this.gift = gift;
    }
    
    //overridden toString() method
    public String toString() { 
        return this.bdayKid + " turns " + this.age + "! They are" + 
            (this.gift ? "" : "not ") + " getting a gift.";
    }
}

public class MainClass{
   public static void main(String []args) {
      ArrayList<Birthday> bdays = getBirthdays();
      printBirthdays(bdays);
   }

   //This method will return a list of birthdays
   public ArrayList<Birthday> getBirthdays() { 
      ArrayList<Birthday> bdays = new ArrayList<Birthday>();
      Scanner scan = new Scanner(System.in);
      
      //create a birthday
      Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
      //add the birthday to arraylist of birthdays
      bdays.add(bday);

      
      return bdays;
   }

   //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         System.out.println(bday);
      }
   }
}

public class MainClass{
   public static void main(String []args) {
      // Instantiate your one and only bdays array
      ArrayList<Birthday> bdays = new ArrayList<Birthday>();
      getBirthdays(bdays);
      printBirthdays(bdays);
   }

   //This method will return a list of birthdays
   public void getBirthdays(ArrayList<Birthday> bdays) { 
      Scanner scan = new Scanner(System.in);
      
      //create a birthday
      Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
      //add the birthday to arraylist of birthdays
      bdays.add(bday);

      // Good practice to close a scanner
      scan.close();
   }

   //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         System.out.println(bday);
      }
   }
}

---

请注意，Java将对象作为引用传递，因此如果您将

**bdays**

传递给

getBirthdays（bdays）

，则此函数中对

bdays

所做的每一项更改也将出现在

main

方法中的原始

bdays

中。更准确地说，它们在内存中是相同的

ArrayList

，变量

bdays

使用两种不同的方法

main

和

getBirthdays（bdays）

只是引用在函数开始时创建的相同

ArrayList

并作为对

getBirthdays

方法的引用传递的不同标识符。查看有关在java中通过引用传递变量的其他相关教程。

您在理解java时遇到了严重问题。首先，你需要理解一个概念，一个变量的范围

如果在函数中创建变量，则变量的作用域仅在函数中

您有标识符为

bdays

的

ArrayList

的不同实例。在

void main

函数中定义的一个实例，以及每次调用

getBirthdays（）

方法时创建的其他实例。在switch语句中，当调用

getBirthdays（）

时，函数将形成并返回一个新的

ArrayList

，而原始的

ArrayList bdays

保持不变。当您调用

printBirthdays（bdays）

时，它将参数作为您在主方法的第一行中创建的数组，而不是

getBirthdays（）

方法返回的新

ArrayList

。因此，它不会打印新的

Brithday

。因此，您可以按如下方式更改代码：

主类

public class Birthday {
    private String bdayKid;
    private int age;
    private boolean gift;
    
    public Birthday(String bdayKid, int age, boolean gift) {
        this.bdayKid = bdayKid;
        this.age = age;
        this.gift = gift;
    }
    
    //overridden toString() method
    public String toString() { 
        return this.bdayKid + " turns " + this.age + "! They are" + 
            (this.gift ? "" : "not ") + " getting a gift.";
    }
}

public class MainClass{
   public static void main(String []args) {
      ArrayList<Birthday> bdays = getBirthdays();
      printBirthdays(bdays);
   }

   //This method will return a list of birthdays
   public ArrayList<Birthday> getBirthdays() { 
      ArrayList<Birthday> bdays = new ArrayList<Birthday>();
      Scanner scan = new Scanner(System.in);
      
      //create a birthday
      Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
      //add the birthday to arraylist of birthdays
      bdays.add(bday);

      
      return bdays;
   }

   //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         System.out.println(bday);
      }
   }
}

public class MainClass{
   public static void main(String []args) {
      // Instantiate your one and only bdays array
      ArrayList<Birthday> bdays = new ArrayList<Birthday>();
      getBirthdays(bdays);
      printBirthdays(bdays);
   }

   //This method will return a list of birthdays
   public void getBirthdays(ArrayList<Birthday> bdays) { 
      Scanner scan = new Scanner(System.in);
      
      //create a birthday
      Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
      //add the birthday to arraylist of birthdays
      bdays.add(bday);

      // Good practice to close a scanner
      scan.close();
   }

   //This method will print a list of birthdays
   public void printBirthdays(ArrayList<Birthday> bdays) {    
      //iterate through each birthday and print out the result 
      for (Birthday bday : bdays) {
         System.out.println(bday);
      }
   }
}

---

请注意，Java将对象作为引用传递，因此如果您将

**bdays**

传递给

getBirthdays（bdays）

，则此函数中对

bdays

所做的每一项更改也将出现在

main

方法中的原始

bdays

中。更准确地说，它们在内存中是相同的

ArrayList

，变量

bdays

使用两种不同的方法

main

和

getBirthdays（bdays）

只是引用在函数开始时创建的相同

ArrayList

并作为对

getBirthdays

方法的引用传递的不同标识符。查看java中通过引用传递变量的其他相关教程。

我认为最好的方法是将getBirthdays（）拆分为printBirthdays（）和AddBirthdays（）。以下是代码，您可以使用自己的逻辑来循环输入：

import java.util.ArrayList;
import java.util.Scanner;

public class MainClass {
    static ArrayList<Birthday> bdays = new ArrayList<>();
    public static void main(String []args) {

        while(true) {
            //Use this method every time you need to add new BDay
            addBirthday();
            //Use this method to print all BDays in ArrayList
            printBirthdays();
        }
    }
    public static void addBirthday(){
        Scanner scan = new Scanner(System.in);
        Birthday bday = new Birthday(scan.nextLine(), scan.nextInt(), scan.nextBoolean());
        bdays.add(bday);
    }
    public static void printBirthdays(){
        for(Birthday b: bdays){
            System.out.println(b);
        }
    }
}

import java.util.ArrayList；
导入java.util.Scanner；
公共类主类{
静态ArrayList bdays=新ArrayList（）；
公共静态void main（字符串[]args）{
while（true）{
//每次需要添加新的BDay时，请使用此方法
添加生日（）；
//使用此方法打印ArrayList中的所有B天
打印生日（）；
}
}
公共静态void addbirth（）{
扫描仪扫描=新扫描仪（System.in）；
生日bday=新生日（scan.nextLine（），scan.nextInt（），scan.nextBoolean（））；
b天。添加（b天）；
}
公共静态vo