英文:
Modify elements of an ArrayList
问题
public class Main {
public static void main(String args[]){
ArrayList<String> aList = new ArrayList<>();
aList.add("aa");
aList.add("bb");
aList.add("cc");
new Main().modifyList(aList);
for(String s: aList){
System.out.println(s);
}
}
public void modifyList(ArrayList<String> aList){
for(int i = 0; i < aList.size(); i++){
String s = aList.get(i);
System.out.println(s);
s = s + "ss";
aList.set(i, s);
}
}
}
英文:
For the below code, I was hoping that elements of my arraylist would be modified but its not. How can I modify the elements
public class Main {
public static void main(String args[]){
ArrayList<String> aList = new ArrayList<>();
aList .add("aa");
aList .add("bb");
aList .add("cc");
new Main().modifyList(aList );
for(String s: aList ){
System.out.println(s);
}
}
public void modifyList(ArrayList<String> aList ){
for(String s: aList){
System.out.println(s);
s = s + "ss";
}
}
}
Its printing
aa
bb
cc
aa
bb
cc
Expected output
aa
bb
cc
aass
bbss
ccss
答案1
得分: 1
public void modifyList(ArrayList<String> aList ){
for(String s: aList){
System.out.println(s);
s = s + "ss";
}
}
字符串是不可变的。所以当你改变 s 时，你创建了一个新的对象，它与 ArrayList 中的那个不同。
因此，你需要遍历数组，并使用 set 方法将旧值替换为新值。
for (int i = 0; i < alist.size(); i++) {
String s = aList.get(i) + "ss";
aList.set(i, s);
}
要简单地追加更改，请执行以下操作：
int len = alist.size();
for (int i = 0; i < len; i++) {
String s = aList.get(i) + "ss";
aList.add(s);
}
打印结果：
> aa bb cc aass bbss ccss
<details>
<summary>英文:</summary>
public void modifyList(ArrayList<String> aList ){
for(String s: aList){
System.out.println(s);
s = s + "ss";
}
}
Strings are immutable. So when you change `s` you are creating a new object that is different than the one in the ArrayList.
So you need to iterate over the array and replace the old value with the new using the `set` method.
for (int i = 0; i < alist.size(); i++) {
String s = aList.get(i) + "ss";
aList.set(i, s);
}
To simply append the changes do the following:
int len = alist.size();
for (int i = 0; i < len; i++) {
String s = aList.get(i) + "ss";
aList.add(s);
}
Prints
> aa bb cc aass bbss ccss
</details>
# 答案2
**得分**: 1
`s = s + "ss"`只会更新局部变量`s`，不会更新列表。
如果你想要更新列表中的元素，可以使用[`ListIterator`][1]和[`set()`][2]方法：
```java
public static void modifyList(List<String> aList) {
for (ListIterator<String> iter = aList.listIterator(); iter.hasNext(); ) {
String s = iter.next();
s = s + "ss";
iter.set(s); // 用新值替换列表中的元素
}
}
在Java 8及以上版本中，你可以使用lambda表达式和replaceAll()方法：
public static void modifyList(List<String> aList) {
aList.replaceAll(s -> s + "ss");
}
无论列表是否很好地处理随机访问，例如列表是LinkedList，这两种方法都能表现良好。
测试
public static void main(String[] args) {
List<String> aList = new ArrayList<>(Arrays.asList("aa", "bb", "cc"));
modifyList(aList);
System.out.println(aList);
}
输出
[aass, bbss, ccss]
英文:
s = s + "ss" only updates the local variable s, it doesn't update the list.
If you want to update elements in a list, use a ListIterator and the set() method:
public static void modifyList(List<String> aList) {
for (ListIterator<String> iter = aList.listIterator(); iter.hasNext(); ) {
String s = iter.next();
s = s + "ss";
iter.set(s); // replace element in the list with new value
}
}
In Java 8+, you can use a lambda expression with the replaceAll() method:
public static void modifyList(List<String> aList) {
aList.replaceAll(s -> s + "ss");
}
Both will perform well even if the list doesn't handle random access well, e.g. if the list is a LinkedList.
Test
public static void main(String[] args) {
List<String> aList = new ArrayList<>(Arrays.asList("aa", "bb", "cc"));
modifyList(aList);
System.out.println(aList);
}
Output
[aass, bbss, ccss]
答案3
得分: 1
问题在于变量s未被使用，且仅在for循环的作用域内可见：
for (String s: aList) {
System.out.println(s);
s = s + "ss"; // 变量's'未被使用
}
你可以使用List::set来替换当前值：
for (int i=0; i<aList.size(); i++) {
String s = aList.get(i);
System.out.println(s);
aList.set(i, s + "ss");
}
... 或者利用 [tag:java-stream] 在 [tag:java-8] 中的优势，将列表映射为新列表：
List<String> newList = aList.stream()
.map(s -> s + "ss")
.collect(Collectors.toList());
英文:
Here is the problem, the variable s is unused and visible only in the scope of the for-loop:
for (String s: aList) {
System.out.println(s);
s = s + "ss"; // the variable 's' is unused
}
Either use List::set to replace the current value:
for (int i=0; i<aList.size(); i++) {
String s = aList.get(i);
System.out.println(s);
aList.set(i, s + "ss");
}
... or use the advantage of [tag:java-stream] as of [tag:java-8] and map the list to a new one:
List<String> newList = aList.stream()
.map(s -> s + "ss")
.collect(Collectors.toList());
答案4
得分: 0
为了解决这个问题，你需要按以下方式更新每个元素：
public void modifyList(ArrayList<String> aList){
for(int i = 0; i < aList.size(); i++){
String s = aList.get(i);
aList.set(i, s + "ss");
}
}
英文:
In order to solve this, you need to update each element as follows:
public void modifyList(ArrayList<String> aList){
for(int i = 0; i < aList.size(); i++){
String s = aList.get(i);
aList.set(i, s+"ss");
}
}
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