Record the pitfalls of Java collection class List

Java basic tutorialColumn introduction Collection class includes two major categories: Map and Collection

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Some high-level programming languages ​​​​now provide the implementation of various out-of-the-box data structures, such as Java programming The collection framework of the language provides various implementations. The collection class includes two major categories: Map and Collection. The List below Collection is one of the collection classes we often use. Many business codes are inseparable from it. Today Let’s take a look at some pitfalls of List lists.

The first pitfall: the List returned by the Arrays.asList method does not support addition and deletion operations

For example, if we execute the following code:

List<String> strings = Arrays.asList("m", "g");
strings.add("h");

will throwjava .lang.UnsupportedOperationException Exception, what is your OS at this time? Why can't elements be added to the returned ArrayList? Can elements be added properly in the future? , and then decisively enable Debug Dafa:

found that the returned ArrayList is not our commonly used java .util.ArrayList, but the inner class java.util.Arrays.ArrayList of Arrays. Enter the method Arrays.asList and the source code is as follows:

public static <T> List<T> asList(T... a) {
    return new ArrayList<>(a);
}

The method returns the static internal class java.util.Arrays.ArrayList of Arrays, Although this class and java.util.ArrayList also inherit from the abstract class java.util.AbstractList, but through the source code of this class, it is found that it does not have any reference to the abstract parent class AbstractList 's add method throws java.lang.UnsupportedOperationException exception by default.

The root cause of this pitfall is that the add method of the strings returned by our call is inherited from the abstract parent classadd method, and the method of the abstract parent class throws the java.lang.UnsupportedOperationException exception by default.

The second pitfall is that the new List returned by the Arrays.asList method and the modification of the original parameter group of the method will affect each other

Arrays.asList method except the aboveDoes not support adding or deleting elements In addition to this pit, there is another pit:

It can be found from the above code that modifications to the original array will affect The new List we obtained through the Arrays.asList method can be found by digging into the source code of java.util.Arrays.ArrayList:

private static class ArrayList<E> extends AbstractList<E>
        implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
        private final E[] a;

        ArrayList(E[] array) {
            a = Objects.requireNonNull(array);
        }
        
        ...
        
     }

The original array is used directly, so we must pay special attention when we use the List obtained by Arrays.asList. Because the array is shared, some unexpected bugs may occur when modifying each other. . One of the standard actions is to use it as a parameter of the ArrayList constructor method to new a List (e.g. List<String> stringList = new ArrayList<>(Arrays.asList(arrays))) or pass Lists.newArrayList in the Guava library, the new List will be returned Decoupled from the original array, they will no longer affect each other.

The third pitfall is that if you directly traverse the List collection to delete elements, an error will be reported.

When you directly traverse the collection elements, adding or deleting elements will report an error. For example, if you execute the following code:

List<String> stringList = Lists.newArrayList("m", "g", "h");
for (String s : stringList) {
    if (Arrays.asList("m", "h").contains(s)) {
        stringList.remove(s);
    }
}

The above code can be compiled normally, but the java.util.ConcurrentModificationException exception will be thrown when executed. Looking at the source code, you can find that the deletion element method remove will modify the collection structure, and also That is, modCount (the number of actual modifications to the collection) will be modified. During the loop, the actual number of modifications to the current List collection modCount will be compared with the iterator modification. The number of expectedModCount, and expectedModCount is the modCount during initialization. If the two are not equal, a ConcurrentModificationException exception will be reported. There are two main solutions: 1. Use the iterator method of ArrayList to traverse, and then call the methods in it. 2. In JDK 1.8, you can use the removeIf method to perform deletion operations.

Finally, a heart-wrenching question: Call the remove method of ArrayList and pass in int basic type numbers and Integer Are the execution results of packaging type numbers the same?

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