String is immutable, StringBuilder is mutable and non-thread-safe, StringBuffer is mutable and thread-safe. 1. Once the content of String is created cannot be modified, it is suitable for a small amount of splicing; 2. StringBuilder is suitable for frequent splicing of single threads and has high performance; 3. StringBuffer is suitable for multi-threaded shared scenarios, but has a slightly lower performance; 4. Reasonably set the initial capacity and avoid using String splicing in loops can improve performance.
The three classes String, StringBuilder and StringBuffer in Java are often confusing, especially when you are just starting out. They are all related to string operations, but the usage and applicable scenarios are quite different. Simply put: String is immutable, StringBuilder is mutable and non-thread-safe, and StringBuffer is mutable and thread-safe . Choosing the right one can improve performance, but choosing the wrong one may cause problems.
Why is String immutable?
String cannot modify content once created. For example, you wrote:
String s = "hello"; s = " world";
This is actually a new object was created, and the original "hello" was not modified. This design is mainly for security and efficiency , such as having hash values ??can be cached, being a HashMap key is more stable, and it can also avoid synchronization problems under multi-threading.
However, this also brings performance problems, especially when strings are frequently spliced, a large number of intermediate garbage objects will be generated. So if you just splice a small amount, it is okay, but if you are processing a large amount of data in a loop, it is not recommended to use String.
StringBuilder: fast speed, suitable for single threading
If you need to frequently splice strings in a single threaded environment, you prefer StringBuilder . It does not create a new object every time it is spliced, but maintains an array of characters internally and expands the capacity dynamically.
For example:
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 1000; i ) {
sb.append(i);
}
String result = sb.toString();This process will not generate a large number of temporary objects, and it is much more efficient than using String.
Its methods are basically common operations such as append, insert, delete, and reverse, which are also very intuitive to use.
StringBuffer: thread safe, but slightly slow
StringBuffer and StringBuilder are almost exactly the same. The only difference is: The StringBuffer method has the synchronized keyword , which means it is thread-safe.
When to use it? StringBuffer is only needed to consider when sharing string buffers in a multithreaded environment . Otherwise, StringBuilder is generally used because synchronized will bring additional overhead.
For example, if the following code is called at the same time by multiple threads, it is recommended to use StringBuffer:
public class SharedBuffer {
private StringBuffer buffer = new StringBuffer();
public void add(String text) {
buffer.append(text);
}
}But if you are sure it is single threaded or not shared, there is no need to spend this extra cost.
Actual selection suggestions
- When more reads, fewer writes, and fewer splicing → Use String
- Frequent splicing, single thread → Use StringBuilder
- Frequent splicing, multi-thread sharing → Use StringBuffer
There are several other points to note:
- Reasonable initial capacity setting can reduce the number of expansions and improve performance
- Don't use String to splice in a loop to "save trouble"
- If it's just a simple string combination, there's no need to force StringBuilder
Basically that's it. The difference between these categories seems simple, but it has a significant impact in actual development, especially in areas with sensitive performance.
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