1. 加号 “+”

   
   

2. String contact() 方法

   
   

3. StringUtils.join() 方法

   
   

4. StringBuffer append() 方法

   
   

5. StringBuilder append() 方法

1. 方法1 加号 “+” 拼接 和 方法2 String contact() 方法 适用于小数据量的操作，代码简洁方便，加号“+” 更符合我们的编码和阅读习惯；

   
   

2. 方法3 StringUtils.join() 方法 适用于将ArrayList转换成字符串，就算90万条数据也只需68ms，可以省掉循环读取ArrayList的代码；

   
   

3. 方法4 StringBuffer append() 方法 和 方法5 StringBuilder append() 方法 其实他们的本质是一样的，都是继承自AbstractStringBuilder，效率最高，大批量的数据处理最好选择这两种方法。

   
   

4. 方法1 加号 “+” 拼接 和 方法2 String contact() 方法 的时间和空间成本都很高（分析在本文末尾），不能用来做批量数据的处理。

package cnblogs.twzheng.lab2;
/** * @author Tan Wenzheng * */import java.util.ArrayList;import java.util.List;
import org.apache.commons.lang3.StringUtils;
public class TestString {
    private static final int max = 100;
    public void testPlus() {        System.out.println(">>> testPlus() <<<");
        String str = "";
        long start = System.currentTimeMillis();
        for (int i = ; i < max; i++) {            str = str + "a";        }
        long end = System.currentTimeMillis();
        long cost = end - start;
        System.out.println("   {str + \"a\"} cost=" + cost + " ms");    }
    public void testConcat() {        System.out.println(">>> testConcat() <<<");
        String str = "";
        long start = System.currentTimeMillis();
        for (int i = ; i < max; i++) {            str = str.concat("a");        }
        long end = System.currentTimeMillis();
        long cost = end - start;
        System.out.println("   {str.concat(\"a\")} cost=" + cost + " ms");    }
    public void testJoin() {        System.out.println(">>> testJoin() <<<");
        long start = System.currentTimeMillis();
        List<String> list = new ArrayList<String>();
        for (int i = ; i < max; i++) {            list.add("a");        }
        long end1 = System.currentTimeMillis();        long cost1 = end1 - start;
        StringUtils.join(list, "");
        long end = System.currentTimeMillis();        long cost = end - end1;
        System.out.println("   {list.add(\"a\")} cost1=" + cost1 + " ms");        System.out.println("   {StringUtils.join(list, \"\")} cost=" + cost                + " ms");    }
    public void testStringBuffer() {        System.out.println(">>> testStringBuffer() <<<");
        long start = System.currentTimeMillis();
        StringBuffer strBuffer = new StringBuffer();
        for (int i = ; i < max; i++) {            strBuffer.append("a");        }        strBuffer.toString();
        long end = System.currentTimeMillis();
        long cost = end - start;
        System.out.println("   {strBuffer.append(\"a\")} cost=" + cost + " ms");    }
    public void testStringBuilder() {        System.out.println(">>> testStringBuilder() <<<");
        long start = System.currentTimeMillis();
        StringBuilder strBuilder = new StringBuilder();
        for (int i = ; i < max; i++) {            strBuilder.append("a");        }        strBuilder.toString();
        long end = System.currentTimeMillis();
        long cost = end - start;
        System.out                .println("   {strBuilder.append(\"a\")} cost=" + cost + " ms");    }}

1. 执行100次， private static final int max = 100;

>>> testPlus() <<<   {str + "a"} cost= ms>>> testConcat() <<<   {str.concat("a")} cost= ms>>> testJoin() <<<   {list.add("a")} cost1= ms   {StringUtils.join(list, "")} cost=20 ms>>> testStringBuffer() <<<   {strBuffer.append("a")} cost= ms>>> testStringBuilder() <<<   {strBuilder.append("a")} cost= ms

2. 执行1000次， private static final int max = 1000;

>>> testPlus() <<<   {str + "a"} cost=10 ms>>> testConcat() <<<   {str.concat("a")} cost= ms>>> testJoin() <<<   {list.add("a")} cost1= ms   {StringUtils.join(list, "")} cost=20 ms>>> testStringBuffer() <<<   {strBuffer.append("a")} cost= ms>>> testStringBuilder() <<<   {strBuilder.append("a")} cost= ms

3. 执行1万次， private static final int max = 10000;

>>> testPlus() <<<   {str + "a"} cost=150 ms>>> testConcat() <<<   {str.concat("a")} cost=70 ms>>> testJoin() <<<   {list.add("a")} cost1= ms   {StringUtils.join(list, "")} cost=30 ms>>> testStringBuffer() <<<   {strBuffer.append("a")} cost= ms>>> testStringBuilder() <<<   {strBuilder.append("a")} cost= ms

4. 执行10万次， private static final int max = 100000;

>>> testPlus() <<<   {str + "a"} cost=4198 ms>>> testConcat() <<<   {str.concat("a")} cost=1862 ms>>> testJoin() <<<   {list.add("a")} cost1=21 ms   {StringUtils.join(list, "")} cost=49 ms>>> testStringBuffer() <<<   {strBuffer.append("a")} cost=10 ms>>> testStringBuilder() <<<   {strBuilder.append("a")} cost=10 ms

5. 执行20万次， private static final int max = 200000;

>>> testPlus() <<<   {str + "a"} cost=17196 ms>>> testConcat() <<<   {str.concat("a")} cost=7653 ms>>> testJoin() <<<   {list.add("a")} cost1=20 ms   {StringUtils.join(list, "")} cost=51 ms>>> testStringBuffer() <<<   {strBuffer.append("a")} cost=20 ms>>> testStringBuilder() <<<   {strBuilder.append("a")} cost=16 ms

6. 执行50万次， private static final int max = 500000;

>>> testPlus() <<<   {str + "a"} cost=124693 ms>>> testConcat() <<<   {str.concat("a")} cost=49439 ms>>> testJoin() <<<   {list.add("a")} cost1=21 ms   {StringUtils.join(list, "")} cost=50 ms>>> testStringBuffer() <<<   {strBuffer.append("a")} cost=20 ms>>> testStringBuilder() <<<   {strBuilder.append("a")} cost=10 ms

7. 执行90万次， private static final int max = 900000;

>>> testPlus() <<<   {str + "a"} cost=456739 ms>>> testConcat() <<<   {str.concat("a")} cost=186252 ms>>> testJoin() <<<   {list.add("a")} cost1=20 ms   {StringUtils.join(list, "")} cost=68 ms>>> testStringBuffer() <<<   {strBuffer.append("a")} cost=30 ms>>> testStringBuilder() <<<   {strBuilder.append("a")} cost=24 ms

1. 其实每次调用contact()方法就是一次数组的拷贝，虽然在内存中是处理都是原子性操作，速度非常快，但是，最后的return语句会创建一个新String对象，限制了concat方法的速度。
   

 

public String concat(String str) {        int otherLen = str.length();        if (otherLen == ) {            return this;        }        int len = value.length;        char buf[] = Arrays.copyOf(value, len + otherLen);        str.getChars(buf, len);        return new String(buf, true);    }

2. StringBuffer 和 StringBuilder 的append方法都继承自AbstractStringBuilder，整个逻辑都只做字符数组的加长，拷贝，到最后也不会创建新的String对象，所以速度很快，完成拼接处理后在程序中用strBuffer.toString()来得到最终的字符串。

/**     * Appends the specified string to this character sequence.     * <p>     * The characters of the {@code String} argument are appended, in     * order, increasing the length of this sequence by the length of the     * argument. If {@code str} is {@code null}, then the four     * characters {@code "null"} are appended.     * <p>     * Let <i>n</i> be the length of this character sequence just prior to     * execution of the {@code append} method. Then the character at     * index <i>k</i> in the new character sequence is equal to the character     * at index <i>k</i> in the old character sequence, if <i>k</i> is less     * than <i>n</i>; otherwise, it is equal to the character at index     * <i>k-n</i> in the argument {@code str}.     *     * @param   str   a string.     * @return  a reference to this object.     */    public AbstractStringBuilder append(String str) {        if (str == null) str = "null";        int len = str.length();        ensureCapacityInternal(count + len);        str.getChars(, len, value, count);        count += len;        return this;    }

/**     * This method has the same contract as ensureCapacity, but is     * never synchronized.     */    private void ensureCapacityInternal(int minimumCapacity) {        // overflow-conscious code        if (minimumCapacity - value.length > )            expandCapacity(minimumCapacity);    }
    /**     * This implements the expansion semantics of ensureCapacity with no     * size check or synchronization.     */    void expandCapacity(int minimumCapacity) {        int newCapacity = value.length * 2 + 2;        if (newCapacity - minimumCapacity < )            newCapacity = minimumCapacity;        if (newCapacity < ) {            if (minimumCapacity < ) // overflow                throw new OutOfMemoryError();            newCapacity = Integer.MAX_VALUE;        }        value = Arrays.copyOf(value, newCapacity);    }

3. 字符串的加号“+” 方法， 虽然编译器对其做了优化，使用StringBuilder的append方法进行追加，但是每循环一次都会创建一个StringBuilder对象，且都会调用toString方法转换成字符串，所以开销很大。
   
   

4. 本文开头的地方统计了时间开销，根据上述分析再想想空间的开销。常说拿空间换时间，反过来是不是拿时间换到了空间呢，但是在这里，其实时间是消耗在了重复的不必要的工作上（生成新的对象，toString方法），所以对大批量数据做处理时，加号“+” 和 contact 方法绝对不能用，时间和空间成本都很高。