用示例替换 Java 中的 HashMap(key，oldValue，newValue)方法

原文:https://www . geesforgeks . org/hashmap-replace key-old value-new value-method-in-Java-with-examples/

由 HashMap 类 实现的 映射界面 的替换(K 键，V oldValue，V newValue) 方法，仅当该键之前映射了指定的旧值时，用于替换指定键的旧值。

语法:

default boolean replace(K key, V oldValue, V newValue)

参数:该方法接受三个参数:

  • key: is the key of the element whose value needs to be replaced.
  • Old value: is the old value of and must be mapped with the provided key.
  • New value: is the new value of and must be mapped with the specified key.

返回值:如果旧值被替换，该方法返回布尔值真，否则假。

异常:如果指定的键或值为空，并且此映射不允许空键或值，则此方法将抛出:

  • null pointerexception , which is thrown if some attributes of the specified key or value prevent it from being stored in this mapping.
  • 非法引数异常。

程序 1:

// Java program to demonstrate
// replace(K key, V oldValue, V newValue) method

import java.util.*;

public class GFG {

    // Main method
    public static void main(String[] args)
    {

        // Create a HashMap and add some values
        HashMap<String, Integer> map
            = new HashMap<>();
        map.put("a", 100);
        map.put("b", 300);
        map.put("c", 300);
        map.put("d", 400);

        // print map details
        System.out.println("HashMap: "
                           + map.toString());

        // provide old value, new value for the key
        // which has to replace it's old value, using
        // replace(K key, V oldValue, V newValue) method
        map.replace("b", 300, 200);

        // print new mapping
        System.out.println("New HashMap: "
                           + map.toString());
    }
}

输出:

HashMap: {a=100, b=300, c=300, d=400}
New HashMap: {a=100, b=200, c=300, d=400}

程序二:

// Java program to demonstrate
// replace(K key, V oldValue, V newValue) method

import java.util.*;

public class GFG {

    // Main method
    public static void main(String[] args)
    {

        // Create a HashMap and add some values
        HashMap<String, Integer> map
            = new HashMap<>();
        map.put("a", 100);
        map.put("b", 300);
        map.put("c", 300);
        map.put("d", 400);

        // print map details
        System.out.println("HashMap: "
                           + map.toString());

        // provide old value, new value for the key
        // which has to replace it's old value,
        // and store the return value in isReplaced using
        // replace(K key, V oldValue, V newValue) method
        boolean isReplaced = map.replace("b", 200, 500);

        // print the value of isReplaced
        System.out.println("Old value for 'b' was "
                           + "replaced: " + isReplaced);

        // print new mapping
        System.out.println("New HashMap: "
                           + map.toString());
    }
}

输出:

HashMap: {a=100, b=300, c=300, d=400}
Old value for 'b' was replaced: false
New HashMap: {a=100, b=300, c=300, d=400}

参考文献:https://docs . Oracle . com/javase/8/docs/API/Java/util/hashmap . html # replace-K-V-V-

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