Why use it cautiously Arrays.asList , sublist of ArrayList?

1. Use Arrays.asList Precautions for

1.1 A pit you might tread in

Let's start with Arrays.asList Use ：

List<Integer> statusList = Arrays.asList(1, 2);
System.out.println(statusList);
System.out.println(statusList.contains(1));
System.out.println(statusList.contains(3));

The output result is shown in the figure below ：

then , Go to statusList Add elements to it 3, As shown below ：

statusList.add(3);
System.out.println(statusList.contains(3));

Expected results , It should be output true, But it was actually thrown out java.lang.UnsupportedOperationException abnormal ：

I can't help but wonder , Just add an element , Why did you throw such an exception , It's not scientific .

1.2 Cause analysis

With this question , Let's take a look at Arrays Static methods provided by class asList Source code ：

The return is ArrayList, Familiar with , Is there any , But take a closer look , You'll find this ArrayList It's not something we use very often ArrayList, Because we usually use ArrayList It's located in java.util Under bag ：

But here ArrayList nevertheless Arrays The inner class of a class ：

It also inherited AbstractList class , Rewriting a lot of methods , For example, the one we used above contains Method , But it didn't rewrite add Method , So we're calling add Method java.lang.UnsupportedOperationException abnormal .

On this point , stay 《 Alibaba Java Development Manual 》 In Taishan version , It is also mentioned that ：

Use tool class Arrays.asList() When converting an array to a collection , You can't use it to modify collection related methods , its add/remove/clear Method will throw UnsupportedOperationException abnormal .

So we are using Arrays.asList You should pay attention to , Avoid trampling .

1.3 summary

Arrays.asList Methods can be used in some simple situations , For example, to quickly declare a collection , Determine whether a value is within the allowable range ：

But don't call after declaration add And so on to modify the set , Otherwise it will be reported java.lang.UnsupportedOperationException abnormal .

2. Use ArrayList Of subList Precautions for

Let's start with subList Simple use ：

List<String> bookList = new ArrayList<>();
bookList.add(" Distant Savior ");
bookList.add(" betrayal ");
bookList.add(" The world of heaven ");
bookList.add(" life ");
bookList.add(" Ordinary world ");

List<String> luyaoBookList = bookList.subList(3, 5);

System.out.println(bookList);
System.out.println(luyaoBookList);

The results are shown in the following figure ：

As you can see from the results ,subList The return is bookList Index from fromIndex（ contain ） To toIndex（ It doesn't contain ） Element collection for .

It's simple to use , It's easy to understand , However, there are still several points to pay attention to , Otherwise, it will cause program errors or exceptions ：

1. Modify the value of the original set element , It affects subsets
2. Modify the structure of the original set , Can cause ConcurrentModificationException abnormal
3. Modify the value of a subset element , Will affect the original set
4. Modify the structure of the subset , Will affect the original set

The above points are in 《 Alibaba Java Development Manual 》 This is what the Taishan version describes ：

2.1 Modify the value of the original set , It affects subsets

such as , Let's modify the original set bookList The value of an element in （ Non structural modification ）：

List<String> bookList = new ArrayList<>();
bookList.add(" Distant Savior ");
bookList.add(" betrayal ");
bookList.add(" The world of heaven ");
bookList.add(" life ");
bookList.add(" Ordinary world ");

List<String> luyaoBookList = bookList.subList(3, 5);

System.out.println(bookList);
System.out.println(luyaoBookList);

//  Modify the value of the original set 
bookList.set(3," Lu yao - life ");

System.out.println(bookList);
System.out.println(luyaoBookList);

The running results are as follows ：

It can be seen that , Although we only modified the original set bookList Value , But it affects subsets luyaoBookList.

2.2 Modify the structure of the original set , Can cause ConcurrentModificationException abnormal

such as , Let's go back to the original bookList Add an element to （ Structural modification ）：

List<String> bookList = new ArrayList<>();
bookList.add(" Distant Savior ");
bookList.add(" betrayal ");
bookList.add(" The world of heaven ");
bookList.add(" life ");
bookList.add(" Ordinary world ");

List<String> luyaoBookList = bookList.subList(3, 5);

System.out.println(bookList);
System.out.println(luyaoBookList);

//  Add elements to the original collection 
bookList.add(" The morning begins at noon ");

System.out.println(bookList);
System.out.println(luyaoBookList);

The running results are as follows ：

It can be seen that , When we add elements to the original set （ Structural modification ） after , When traversing subsets , It happened. ConcurrentModificationException abnormal .

matters needing attention ： The above exception does not occur when adding elements , But after adding elements , What happens when traversing a subset .

On this point , stay 《 Alibaba Java Development Manual 》 This is what the Taishan version describes ：

2.3 Modify the value of the subset , Will affect the original set

such as , We modify subsets luyaoBookList The value of an element in （ Non structural modification ）：

List<String> bookList = new ArrayList<>();
bookList.add(" Distant Savior ");
bookList.add(" betrayal ");
bookList.add(" The world of heaven ");
bookList.add(" life ");
bookList.add(" Ordinary world ");

List<String> luyaoBookList = bookList.subList(3, 5);

System.out.println(bookList);
System.out.println(luyaoBookList);

//  Modify the value of the subset 
luyaoBookList.set(1," Lu yao - Ordinary world ");

System.out.println(bookList);
System.out.println(luyaoBookList);

The running results are as follows ：

It can be seen that , Although we only modified the subset luyaoBookList Value , But it affects the original set bookList.

2.4 Modify the structure of the subset , Will affect the original set

such as , Let's go to the subset luyaoBookList Add an element to （ Structural modification ）：

List<String> bookList = new ArrayList<>();
bookList.add(" Distant Savior ");
bookList.add(" betrayal ");
bookList.add(" The world of heaven ");
bookList.add(" life ");
bookList.add(" Ordinary world ");

List<String> luyaoBookList = bookList.subList(3, 5);

System.out.println(bookList);
System.out.println(luyaoBookList);

//  Add elements to the subset 
luyaoBookList.add(" The morning begins at noon ");

System.out.println(bookList);
System.out.println(luyaoBookList);

The running results are as follows ：

It can be seen that , When we add elements to a subset （ Structural modification ） after , It affects the original set bookList.

2.5 Cause analysis

First , Let's take a look at subList Method comments , Find out what it does ：

Returns a view of the portion of this list between the specified {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.

Translation means ：

Returns the specified {@code fromIndex}( contain ) and {@code toIndex}( exclude ) Between the list parts of the view .

then , Let's take a look at the source code ：

public List<E> subList(int fromIndex, int toIndex) {
    subListRangeCheck(fromIndex, toIndex, size);
    return new SubList(this, 0, fromIndex, toIndex);
}

You can see , It calls for SubList Class constructor , The source code of the constructor is shown in the figure below ：

It can be seen that ,SubList Class is ArrayList The inner class of , The constructor does not recreate a new ArrayList, So modify the values of the elements of the original set or subset , It will affect each other .

2.6 summary

ArrayList Of subList Method , It returns a subset of the original set （ View ）, Unstructured modification of the values of elements in any set , Will influence each other , When modifying the original set structurally , Will be submitted to the ConcurrentModificationException abnormal , When modifying subsets structurally , Will affect the original set , So pay attention to the use of , Avoid program errors or exceptions .

3. Reference resources

Use caution ArrayList Medium subList Method

《 Alibaba Java Development Manual 》 Taishan Edition

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