Java 如何打印数组列表中的元素
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How to print elements from an array list
提问by Fuzail Gilani
I've looked online for help so far and nothing is working for me. This is what I have right now, can you please tell me what is wrong with it?
到目前为止，我一直在网上寻求帮助，但对我没有任何帮助。这就是我现在所拥有的，你能告诉我它有什么问题吗？
import java.util.*;
导入 java.util.*;
class employeeProgram{
public static void main(String[] args){
for (int count = 0; count < 1; ){
Scanner input = new Scanner(System.in);
ArrayList<String> employeeNames = new ArrayList<String>();
String commnd;
print("What would you like to do? (Add, Remove, List, Exit): ");
commnd = input.next();
if ("Add".equals(commnd)){
print("Enter the name of the employee you'd like to add to the list: ");
employeeNames.add(input.next());
} else if ("Remove".equals(commnd)){
print("Enter the name of the employee you'd like to remove from the list: ");
employeeNames.remove(input.next());
} else if ("List".equals(commnd)){
for (int j = 0; j < employeeNames.size(); j++){
println(employeeNames.get(j));
println(j);
}
//println(employeeNames);
} else if ("Exit".equals(commnd)){
input.close();
break;
} else {
println("Invalid command.");
}
}
}
public static void println(Object x){
System.out.println(x);
}
public static void print(Object x){
System.out.print(x);
}
}
The for loop isn't beginning, I know that much because the "println(j)" wasn't working either. When I tried the part that I've commented out, all I got from typing list was "[]" and nothing else. I'm a beginner, and this is my first program using ArrayLists, so thanks in advance to anyone who decides to help me out :)
for 循环还没有开始，我知道很多是因为“println(j)”也没有工作。当我尝试我注释掉的部分时，我从输入列表中得到的只是“[]”而没有别的。我是一个初学者，这是我第一个使用 ArrayLists 的程序，所以提前感谢任何决定帮助我的人:)
回答by Juned Ahsan
Define your employeeNamesList outside the first forloop. ArrayList is getting re-intialized with every iteration of the loop.
employeeNames在第一个for循环之外定义您的列表。ArrayList 会随着循环的每次迭代重新初始化。
回答by Hello_Everyone
for (int count = 0; count < 1; count++)
for (int count = 0; count < 1; count++)
回答by Husman
You need to define your arraylist outsode of your loop, otherwise every iteration creates a new arrayList and the previous changes made to the previous arraylist is garbage collected.
您需要在循环外定义 arraylist，否则每次迭代都会创建一个新的 arrayList，并且之前对之前的 arraylist 所做的更改将被垃圾收集。
import java.util.*;
class employeeProgram{
public static void main(String[] args){
ArrayList<String> employeeNames = new ArrayList<String>();
for (int count = 0; count < 1; ){
Scanner input = new Scanner(System.in);
String commnd;
print("What would you like to do? (Add, Remove, List, Exit): ");
commnd = input.next();
if ("Add".equals(commnd)){
print("Enter the name of the employee you'd like to add to the list: ");
employeeNames.add(input.next());
} else if ("Remove".equals(commnd)){
print("Enter the name of the employee you'd like to remove from the list: ");
employeeNames.remove(input.next());
} else if ("List".equals(commnd)){
for (int j = 0; j < employeeNames.size(); j++){
println(employeeNames.get(j));
println(j);
}
//println(employeeNames);
} else if ("Exit".equals(commnd)){
input.close();
break;
} else {
println("Invalid command.");
}
}
}
public static void println(Object x){
System.out.println(x);
}
public static void print(Object x){
System.out.print(x);
}
}
Also, there are a few basic changes I would make to this program, first of all - a for loop should be used where you know how many iterations you will do. In your case, a while loop (or better still a do-while loop) is better suited.
另外，我会对这个程序做一些基本的改变，首先 - 应该使用 for 循环，在那里你知道你将进行多少次迭代。在您的情况下，while 循环（或更好的 do-while 循环）更适合。
Finally creating one liner functions to encapsulate system.out.print and println seem more effort than is necessary.
最后创建一个 liner 函数来封装 system.out.print 和 println 似乎比必要的工作要多。
回答by tom
You are creating a new ArrayList everytime you ask a question. Create it outside of the initial loop.
每次您提出问题时，您都在创建一个新的 ArrayList。在初始循环之外创建它。
ArrayList<String> employeeNames = new ArrayList<String>();
回答by henderso
I would try something else instead of the for loop:
我会尝试其他方法而不是 for 循环：
Although what you have here might technically work, for...loops are for deterministic processes. Here you are using it as an infinite loop that you hope to force break at a later point. This is dangerous and could lead to bugs.
尽管您在这里的内容在技术上可能有效，但 for... 循环适用于确定性过程。在这里，您将其用作无限循环，希望稍后强制中断。这是危险的，可能会导致错误。
I'd change this:
我会改变这个：
for (int count = 0; count < 1; ){
to this:
对此：
bool done = false;
while(!done) {
Then, later, when you want to break out. issue this line:
然后，稍后，当您想突破时。发出这一行：
done = true;
