What is the implementation mechanism of the Java String class equals method? What are the issues worth noting in the JDK18 environment?

In-depth implementation mechanism of Java String class equals method: Detailed analysis under JDK 18

The equals() method of String class in Java is the core of string comparison. This article will explore its implementation mechanism under JDK 18 in depth and analyze some details worth paying attention to in the source code.

Source code analysis and problem discussion

The implementation of equals() method of String class contains some optimization strategies, such as compact_strings and coder fields. compact_strings is a static final boolean value indicating whether compact string storage is enabled; coder records the encoding method of the string (Latin-1 or UTF-16).

Let's analyze the key code snippets:

 return (anobject instanceof String astring)
       && (!compact_strings || this.coder == astring.coder)
       && StringLatin1.equals(value, astring.value);

This code first checks whether anobject is String instance. Then, it determines whether it is necessary to check encoding consistency based on the value of compact_strings . If compact_strings is false , the encoding difference is ignored; otherwise, it compares whether this.coder and astring.coder are the same. Finally, StringLatin1.equals() method performs the actual character comparison.

Question 1: The "loop operation" phenomenon that occurs during debugging

During the debugging process, a phenomenon similar to "cycle operation" may be observed. This is not a real loop, but an illusion caused by factors such as debuggers or encoding conversions. The comparison process inside StringLatin1.equals() method may involve multiple function calls or intermediate steps, which looks like a loop in the debugger.

Question 2: Display of different parameter values

The parameter values ​​after “a”.equals(new String("a")) and “a”.equals("a") are passed to equals() method may be different. This is related to how strings are created and internal representations. The literal string "a" may be optimized at compile time, while new String("a") creates a new string object. This may result in different internal representations observed in the debugger, such as different value arrays or coder values. This does not affect the comparison result, because equals() method will eventually compare the actual character content of the string.

The meaning of compact_strings and coder

compact_strings and coder are mechanisms introduced by Java to optimize string storage and compare efficiency. Enabling compact string storage ( compact_strings is true ) can save memory, but requires checking encoding consistency when comparing. coder field records encoding method to ensure correct character comparisons in compact storage mode.

Summarize

The implementation of equals() method of the Java String class reflects the balance between efficiency and compatibility. While improving performance, compact_strings and coder mechanisms also increase the complexity of understanding the source code. In JDK 18, understanding these details can help better grasp the underlying mechanisms of Java string processing and avoid possible misunderstandings during debugging. The difference in the "loop" and parameter values ​​that occur during the debugging process are phenomena from the perspective of the debugger and do not mean that there is a problem equals() method itself. The final comparison results are still accurate and reliable.

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