如何用 Java 将集合设为只读？
原文:https://www . geesforgeks . org/how-to-make-a-collection-只读 java/
Collections 类用作管理数据的数据结构。我们可以在列表、设置或映射对象中添加、删除、提取和更新数据。Collections 类有这些操作的默认方法。我们可以很容易地使用那些方法。默认情况下，当我们创建集合类的对象时，它将是可读和可写的。
只读集合:要将集合的对象设为只读，我们需要限制一个对象在其中添加、删除或更新数据。唯一的操作是获取数据。
对于不同的集合类型，Java 有不同的方法，比如未修改类集合()、未修改类集合()、未修改类集合() e.t.c .所有的方法都是在 java.util.Collections 类中预定义的。未修改的集合()是一个制作只读集合的通用方法。我们需要为此引用 Collections 类。如果我们有一个设置接口的对象，我们可以使用不可修改的属性设置()使其只读。
示例 1: 下面的代码展示了如何使列表不可修改。
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to make Collections Read-Only
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class GFG {
public static void main(String[] args)
{
// List of Integer
List<Integer> numbers = new ArrayList<>();
// List have 1 to 10 numbers
for (int i = 1; i <= 10; i++) {
numbers.add(i);
}
// Iterate on the stream of integers and
// print them
numbers.stream().forEach(System.out::print);
// Now we are adding one more element
numbers.add(11);
// Removing element from the list
numbers.remove(8);
// Updating List¶
numbers.set(4, 4);
System.out.println(
"\nAfter Performing Some Operations");
numbers.stream().forEach(System.out::print);
System.out.println(
"\nHence By default Collections object is Readable and Writable");
// Now making Read-Only List
// Using unmodifiableList() method.
try {
numbers = Collections.unmodifiableList(numbers);
// This line will generate an Exception
numbers.remove(11);
}
catch (UnsupportedOperationException
unsupportedOperationException) {
System.out.println(
"Exceptions is "
+ unsupportedOperationException);
}
finally {
System.out.println(numbers.get(3));
System.out.println(
"Now list is only Read-Only");
}
}
}
Output
12345678910
After Performing Some Operations
123446781011
Hence By default Collections object is Readable and Writable
Exceptions is java.lang.UnsupportedOperationException
4
Now list is only Read-Only
以上是如何将列表设为只读的示例。在成为只读之前，我们可以执行 CRUD 操作，但是在成为只读列表之后，set()、add()和 remove()方法将生成异常。我们现在只能从列表中获取数据。
示例 2: 下面的代码展示了如何使设置不可修改。
Java
// Java Program to make
// Set Interface Object Read-Only
import java.util.Set;
import java.util.HashSet;
import java.util.Collections;
class GFG {
public static void main(String[] args)
{
// Set of Integer
Set<Integer> numbers = new HashSet<Integer>();
// Set have 1 to 10 numbers
for (int i = 1; i <= 5; i++) {
numbers.add(i);
}
// print the integers
numbers.stream().forEach(System.out::print);
// Removing element from the list
numbers.remove(5);
System.out.println("\nAfter Performing Operation");
numbers.stream().forEach(System.out::print);
System.out.println(
"\nSet is also By Default Readable and Writable");
// Now making Read-Only Set
// Using unmodifiableSet() method.
try {
numbers = Collections.unmodifiableSet(numbers);
// This line will generate an Exception
numbers.remove(4);
}
catch (UnsupportedOperationException
unsupportedOperationException) {
System.out.println(
"Exceptions is "
+ unsupportedOperationException);
}
finally {
System.out.println(numbers.contains(3));
System.out.println("Now Set is Read-Only");
}
}
}
输出
12345
After Performing Operation
1234
Set is also By Default Readable and Writable
Exceptions is java.lang.UnsupportedOperationException
true
Now Set is Read-Only
在上例中，我们将设置为只读。我们可以使用未修改的集合()使集合对象只读，而要使地图只读，我们可以使用未修改的勒马()方法。
| way | Description | | --- | --- | | [Static < T > Set < T > Unmodified Set (Set c)](https://www.google.com/url?client=internal-element-cse&cx=009682134359037907028:tj6eafkv_be&q=https://www.geeksforgeeks.org/collections-unmodifiablecollection-method-in-java-with-examples/&sa=U&ved=2ahUKEwjJ-qvh-4jtAhVzzTgGHQ3pAEIQFjAAegQIAhAC&usg=AOvVaw29Q-BrvBoSY76zTcPshzn7) | This method accepts any collection object and returns the unmodifiable view of the specified collection. | | [Static < T > List < T > Cannot be modified (List list extension)](https://www.google.com/url?client=internal-element-cse&cx=009682134359037907028:tj6eafkv_be&q=https://www.geeksforgeeks.org/collections-unmodifiablelist-method-in-java-with-examples/&sa=U&ved=2ahUKEwjJ-qvh-4jtAhVzzTgGHQ3pAEIQFjABegQICRAC&usg=AOvVaw0zhQbejOyOmBwB5WoWawo-) | This method accepts an object of the list interface and returns its unmodifiable view. | | [Static < K, V > Map < K, V > The map (map m)](https://www.google.com/url?client=internal-element-cse&cx=009682134359037907028:tj6eafkv_be&q=https://www.geeksforgeeks.org/collections-unmodifiablemap-method-in-java-with-examples/&sa=U&ved=2ahUKEwjJ-qvh-4jtAhVzzTgGHQ3pAEIQFjADegQIBxAC&usg=AOvVaw2ynCZsSigSJqUEKFlKw4an) | This method accepts an object of the Map interface and returns its unmodifiable view. | | [Static < T > Setting < T > cannot be modified (setting s)](https://www.google.com/url?client=internal-element-cse&cx=009682134359037907028:tj6eafkv_be&q=https://www.geeksforgeeks.org/collections-unmodifiableset-method-in-java-with-examples/&sa=U&ved=2ahUKEwjJ-qvh-4jtAhVzzTgGHQ3pAEIQFjAGegQIBRAC&usg=AOvVaw3AdHieOG1AUm3lLLhNuC1-) | This method accepts an object of the Set interface and returns its unmodifiable view ... | | [Static < K, V > SortedMap [T V > m)](https://www.google.com/url?client=internal-element-cse&cx=009682134359037907028:tj6eafkv_be&q=https://www.geeksforgeeks.org/collections-unmodifiablesortedmap-method-in-java-with-examples/&sa=U&ved=2ahUKEwjK57zR_IjtAhVzzTgGHQ3pAEI4ChAWMAF6BAgIEAI&usg=AOvVaw2rZQvfiG6-b9tnOtkvkcr9) | This method accepts an object of [SortedMap](https://www.google.com/url?client=internal-element-cse&cx=009682134359037907028:tj6eafkv_be&q=https://www.geeksforgeeks.org/sortedmap-java-examples/&sa=U&ved=2ahUKEwiP5YHt_IjtAhXayDgGHa4MBDAQFjAAegQIBBAC&usg=AOvVaw1bsza0E2Mtqo-nVt0xWVtO) interface and returns its unmodifiable view. | | Static < T > Sort set < T > Unmodifiable sort set (sorted set < T > s) | This method accepts 【 T47 |版权属于：月萌API www.moonapi.com，转载请注明出处
