Java字符串拼接的五种方法及性能比较分析(从执行100次到90万次)

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 = 0; 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 = 0; 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 = 0; 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 = 0; 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 = 0; 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");
    }
}

>>> testPlus() <<<
   {str + "a"} cost=0 ms
>>> testConcat() <<<
   {str.concat("a")} cost=0 ms
>>> testJoin() <<<
   {list.add("a")} cost1=0 ms
   {StringUtils.join(list, "")} cost=20 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=0 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=0 ms

>>> testPlus() <<<
   {str + "a"} cost=10 ms
>>> testConcat() <<<
   {str.concat("a")} cost=0 ms
>>> testJoin() <<<
   {list.add("a")} cost1=0 ms
   {StringUtils.join(list, "")} cost=20 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=0 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=0 ms

>>> testPlus() <<<
   {str + "a"} cost=150 ms
>>> testConcat() <<<
   {str.concat("a")} cost=70 ms
>>> testJoin() <<<
   {list.add("a")} cost1=0 ms
   {StringUtils.join(list, "")} cost=30 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=0 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=0 ms

>>> 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

>>> 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

>>> 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

>>> 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

    public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
        char buf[] = Arrays.copyOf(value, len + otherLen);
        str.getChars(buf, len);
        return new String(buf, true);
    }

    /**
     * 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(0, 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 > 0)
            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 < 0)
            newCapacity = minimumCapacity;
        if (newCapacity < 0) {
            if (minimumCapacity < 0) // overflow
                throw new OutOfMemoryError();
            newCapacity = Integer.MAX_VALUE;
        }
        value = Arrays.copyOf(value, newCapacity);
    }