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1. What is the Java wrapper class caching mechanism?
The wrapper class (Wrapper Class) in Java exists to convert basic data types into objects. In Java, each basic data type has a corresponding wrapper class, such as Integer, Double, etc.
The Java wrapper class caching mechanism means that under certain circumstances, Java will cache wrapper class objects within a certain range to improve performance and save memory space.
2. Why is Java wrapper class caching mechanism needed?
Using wrapper classes can make basic data types have object-oriented characteristics, and it also facilitates the storage of basic data types in collection classes. However, since wrapper classes are objects, they take up more memory space than basic data types.
In order to solve this problem, Java introduced a wrapper class caching mechanism, which reduces the cost of repeatedly creating objects by caching wrapper class objects within a certain range, thereby improving performance and saving memory space.
3. Implementation principle of Java wrapper class caching mechanism
The Java wrapper class caching mechanism is implemented through static member variables. In the five packaging classes of Integer, Long, Short, Byte, and Character, a static array cache[] is defined for caching commonly used values.
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Integer class: Integers between -128 and 127 are cached by default. -
Long class: Long integers between -128 and 127 are cached by default. -
Short class: Short integers between -128 and 127 are cached by default. -
Byte class: Bytes between -128 and 127 are cached by default. -
Character class: Characters between 0 and 127 are cached by default.
当使用 valueOf()方法创建包装类对象时，会先检查该值是否在缓存范围内。如果是，则直接返回缓存中的对象；否则，创建一个新的对象并放入缓存中。
4. Java 包装类缓存机制的使用示例
Integer a = Integer.valueOf(100); // 缓存中不存在，创建新对象
Integer b = Integer.valueOf(100); // 缓存中存在，直接返回缓存对象
System.out.println(a == b); // 输出true，表示a和b引用同一个对象
上述示例中，通过调用Integer.valueOf()方法创建两个 Integer 对象。由于 100 在缓存范围内，第二次创建时直接返回了缓存中的对象，所以 a 和 b 引用同一个对象，输出结果为 true。
5. Java 包装类缓存机制的优点
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提高性能：避免重复创建相同数值的包装类对象，减少了内存开销和垃圾回收的压力。 -
节省内存空间：对于常用的数值，在缓存范围内的包装类对象可以被多个引用共享，减少了内存占用。
6. Java 包装类缓存机制的缺点
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缓存范围有限：只有在特定范围内的数值才会被缓存，超出范围仍然会创建新对象。 -
自动装箱和拆箱的性能损耗：自动装箱和拆箱操作可能导致频繁的包装类对象创建和销毁，影响性能。
7. Java 包装类缓存机制的使用注意事项
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不要依赖包装类缓存机制进行比较：由于缓存机制的存在，相同数值的包装类对象并不一定引用同一个对象。因此，在比较两个包装类对象时，应该使用 equals()方法而不是"=="运算符。 -
超出缓存范围时需要额外注意：当数值超出缓存范围时，每次调用 valueOf()方法都会创建新的对象，这可能会导致意想不到的结果。
8. 总结
Java 包装类缓存机制通过缓存常用的包装类对象，提高了性能和节省了内存空间。它的实现原理是通过静态数组来缓存对象，并在创建对象时先检查是否在缓存范围内。尽管具有一定的局限性和性能损耗，但在合适的场景下，包装类缓存机制仍然是一个有用的优化手段。
参考资料
首发博客地址: https://blog.zysicyj.top/
[2]面试题手册: https://store.amazingmemo.com/chapterDetail/1685324709017001
[3]系列文章地址: https://blog.zysicyj.top/categories/技术文章/后端技术/系列文章/面试题精讲/
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