在本文中，我们将学习 Java ConcurrentHashMap 类的 computeIfAbsent() 方法。
找到 computeIfAbsent 方法的 Java 文档。

V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)

1. 如果指定的键不存在，mappingFunction 会计算一个值并将这个新的键/值添加到映射中。
2. 如果指定的键已经存在，则不计算 mappingFunction 并且映射中没有变化。
3. 如果指定的键不存在，并且 mappingFunction 将值计算为 null，则不会将此新键/值添加到映射中。
4. 整个方法调用以原子方式执行。
5. 映射函数在计算过程中不得修改此映射。

6。参数 ：
key - 关联值的键。
mappingFunction - 计算值的函数。

7。返回：计算后的新值或 null。

8。异常：
NullPointerException：如果指定的键或提供的 mappingFunction 为空。
IllegalStateException：如果计算是递归的。
RuntimeException：如果 mappingFunction 抛出 RuntimeException。

示例-1

ComputeIfAbsent1.java

package com.concretepage;
import java.util.concurrent.ConcurrentHashMap;

public class ComputeIfAbsent1 {
  public static void main(String[] args) {
	ConcurrentHashMap<Integer, String> conMap = new ConcurrentHashMap<>();
	conMap.put(10, "Varanasi");
	conMap.put(20, "Prayag");
	conMap.put(30, "Ayodhya");
	
	System.out.println("--- key is present ---");
	String v1 = conMap.computeIfAbsent(20, k -> k + " - Kanyakumari");
	System.out.println("v1: " + v1);
	System.out.println(conMap);	// No change	

	System.out.println("--- key is not present but value is null ---");
	String v2 = conMap.computeIfAbsent(40, k -> null);
	System.out.println("v2: " + v2);
	System.out.println(conMap); // No change
	
	System.out.println("--- key is not present ---");
	String v3 = conMap.computeIfAbsent(50, k -> k + " - Kanyakumari");
	System.out.println("v3: " + v3);
	System.out.println(conMap); // New entry added
  }
}

输出

--- key is present ---
v1: Prayag
{20=Prayag, 10=Varanasi, 30=Ayodhya}
--- key is not present but value is null ---
v2: null
{20=Prayag, 10=Varanasi, 30=Ayodhya}
--- key is not present ---
v3: 50 - Kanyakumari
{50=50 - Kanyakumari, 20=Prayag, 10=Varanasi, 30=Ayodhya}

示例-2

ComputeIfAbsent2.java

package com.concretepage;
import java.util.concurrent.ConcurrentHashMap;

public class ComputeIfAbsent2 {
  public static void main(String[] args) {
	ConcurrentHashMap<Integer, Integer> conMap = new ConcurrentHashMap<>();
	conMap.put(10, 100);
	conMap.put(20, 200);
	conMap.put(30, 300);
	
	System.out.println("--- key is present ---");
	Integer v1 = conMap.computeIfAbsent(20, k -> k * 10);
	System.out.println("v1: " + v1);
	System.out.println(conMap);	// No change	

	System.out.println("--- key is not present but value is null ---");
	Integer v2 = conMap.computeIfAbsent(40, k -> null);
	System.out.println("v2: " + v2);
	System.out.println(conMap); // No change
	
	System.out.println("--- key is not present ---");
	Integer v3 = conMap.computeIfAbsent(50, k -> k * 10);
	System.out.println("v3: " + v3);
	System.out.println(conMap); // New entry added
  }
}

输出

--- key is present ---
v1: 200
{20=200, 10=100, 30=300}
--- key is not present but value is null ---
v2: null
{20=200, 10=100, 30=300}
--- key is not present ---
v3: 500
{50=500, 20=200, 10=100, 30=300}