Java basics - comparison operation

• Double equal sign == Compare basic data types, only compare values, regardless of precision and type
• Double equal sign == compares reference data types, compares memory addresses, and does not apply
• You can use the .equals method to compare strings, which compares the contents of strings
• The double equal sign == comparing the wrapper class also compares the memory address. If it is a value reserved in the cache, there is no need to create a new wrapper object, and the wrapper objects with equal values ​​are compared to true with a double equal sign. If the value exceeds the cache, the wrapper will automatically create a new wrapper object, and the values ​​​​are equal at this time Wrapper class objects with double equal signs compare to false
• In order to avoid this singularity, the comparison of wrapper class objects generally uses the .equals method

String s1 = "abc"; // java做了优化，这样声明会将对象放在字符串常量池中，如果有一样的字符串，和它公用一块内存
String s2 = "abc"; // 两个"abc"是相同的内存地址
System.out.println(s1 == s2); // 结果为true
String s3 = new String("abc"); // 这样声明一个新对象，与s1不在同一个内存中
System.out.println(s1 == s3); // 结果为false
System.out.println(s1.equals(s3)); // 结果为true,equals方法比较的是字符串的内容
Integer i1 = 128; // 简易写法，实际上调用了valueOf方法
Integer i2 = Integer.valueOf(128); // 完整写法，valueOf方法中有缓存操作，范围-128~127
System.out.println(i1 == i2); // 结果为false，因为128超出了缓存范围，创建了新的包装类对象
System.out.println(i1.equals(i2)); // 结果为true，比较的是内容