How to use contains in java

The contains() method in Java checks whether a specific element or substring exists in a collection or string, and returns true to indicate inclusion, otherwise it returns false. It works with a variety of comparison and search operations on types such as List, Set, Map, and String.

contains() usage in Java

In Java, contains() method Used to check whether a set or string contains a specific element or substring. It is widely used for various comparison and search operations.

Syntax

boolean contains(Object element)

Parameters

• element - The element or child to search for String

Return value

• Returns true if the collection or string contains the element or substring ; Otherwise, return false.

Usage example

List

List<String> names = new ArrayList<>();
names.add("John");
names.add("Mary");
names.add("Bob");

if (names.contains("John")) {
    // John 已存在于列表中
}

Set

Set<Integer> numbers = new HashSet<>();
numbers.add(1);
numbers.add(2);
numbers.add(3);

if (numbers.contains(2)) {
    // 集合中包含数字 2
}

Map

Map<String, Integer> ages = new HashMap<>();
ages.put("John", 30);
ages.put("Mary", 25);
ages.put("Bob", 35);

if (ages.containsKey("John")) {
    // John 已存在于映射中
}

String

String str = "Hello World";

if (str.contains("World")) {
    // 字符串中包含子字符串 "World"
}

Notes

• contains The () method iterates over the entire collection or string, so it may be less efficient, especially if the collection or string is large.
• For primitive types (such as int, char, etc.), you can use the equals() method for comparison instead of contains(). The
• contains() method behaves differently with null values ​​depending on the collection type. For example, List and Set will treat null as a valid element, but Map will not.

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