【JAVA基础】String、StringBuilder和StringBuffer的区别——巨详细

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    // String内部使用char数组来存储数据
    private final char value[];
​
    // ...
}

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence {
    
    // 数组最大长度
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8
​
    // ... 其他方法
​
    // 示例方法：添加字符串
    public StringBuilder append(String str) {
        // 这里的super指向AbstractStringBuilder类
        super.append(str);
        return this;
    }
    
    // ... 其他方法继续
}
​
abstract class AbstractStringBuilder {
    char[] value;
    int count;
​
    // 实际扩展数组和添加内容在这个类中实现
    public void ensureCapacity(int minimumCapacity) {
        if (minimumCapacity > 0)
            ensureCapacityInternal(minimumCapacity);
    }
    
    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }
    
    private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int newCapacity = (value.length << 1) + 2;
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }
    
    private int hugeCapacity(int minCapacity) {
        if (Integer.MAX_VALUE - minCapacity < 0) { // overflow
            throw new OutOfMemoryError();
        }
        return (minCapacity > MAX_ARRAY_SIZE)
            ? minCapacity : MAX_ARRAY_SIZE;
    }
​
    public AbstractStringBuilder append(String str) {
        if (str == null) {
            // ... 处理null字符串的情况
        }
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }
    
    // ... 其他方法
}

 public final class StringBuffer
    extends AbstractStringBuilder
    implements Serializable, CharSequence
{
    ...
    private transient char[] toStringCache;
    ...
    @Override
    public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }
    ...
}

long t1 = System.currentTimeMillis();
String str = "hollis";
for (int i = 0; i < 50000; i++) {
    String s = String.valueOf(i);
    str += s;
}
long t2 = System.currentTimeMillis();
System.out.println("+ cost:" + (t2 - t1));

long t1 = System.currentTimeMillis();
String str = "hollis";
for(int i = 0; i < 50000; i++)
{
    String s = String.valueOf(i);
    str = (new StringBuilder()).append(str).append(s).toString();
}
​
long t2 = System.currentTimeMillis();
System.out.println((new StringBuilder()).append("+ cost:").append(t2 - t1).toString());

package org.example;
​
public class Main {
    public static void main(String[] args) {
        testStringAdd();
        testStringBuilderAdd();
    }
​
    static void testStringAdd() {
        Runtime runtime = Runtime.getRuntime();
​
        long usedMemoryBefore = runtime.totalMemory() - runtime.freeMemory();
        long startTime = System.currentTimeMillis();
​
        String result = "";
        for (int i = 0; i < 10000; i++) {
            result += "some text";
        }
​
        long endTime = System.currentTimeMillis();
        System.out.println("String concatenation with + operator took: " + (endTime - startTime) + " milliseconds");
        long usedMemoryAfter = runtime.totalMemory() - runtime.freeMemory();
        System.out.println("Memory used for String concatenation: " + (usedMemoryAfter - usedMemoryBefore) + " bytes");
    }
​
    static void testStringBuilderAdd() {
        Runtime runtime = Runtime.getRuntime();
​
        long usedMemoryBefore = runtime.totalMemory() - runtime.freeMemory();
        long startTime = System.currentTimeMillis();
​
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < 10000; i++) {
            builder.append("some text");
        }
​
        long endTime = System.currentTimeMillis();
        System.out.println("String concatenation with StringBuilder took: " + (endTime - startTime) + " milliseconds");
​
        long usedMemoryAfter = runtime.totalMemory() - runtime.freeMemory();
        System.out.println("Memory used for String concatenation: " + (usedMemoryAfter - usedMemoryBefore) + " bytes");
    }
}

String concatenation with + operator took: 669 milliseconds
Memory used for String concatenation: 619980944 bytes
String concatenation with StringBuilder took: 0 milliseconds
Memory used for String concatenation: 0 bytes

String result = "";
for (int i = 0; i < 10000; i++) {
    result = (new StringBuilder()).append("some text").toString();
}