package com.example.demo.test;
/**
* <p>
* <code>StringBuilderTest</code>
* </p>
* Description: JDK 1.8
*
* @author Jalen
* @date 2019/8/26 9:41
*/
public class StringBuilderTest {
public static void main(String[] args) {
//createString();
//createEmptyStringBuilder();
createNonEmptyStringBuilder();
}
private static void createString(){
//编译期优化，直接将string赋值为"abc"，可以使用javap -verbose xxx.class查看
String string = "a" + "b" + "c";
System.out.println(string);
//使用+号连接字符串等同于使用StringBuilder的append方法拼接字符串，
//可以使用javap -verbose xxx.class查看字节码
String a = "a";
String b = "b";
String c = "c";
String str = a + b + c;
System.out.println(str);
}
/**
* 创建容量为空的StringBuilder
* 调用链1：
* {@link java.lang.StringBuilder#StringBuilder}
* 这里调用父类构造方法，同时赋值capacity为默认值16
* {@link java.lang.AbstractStringBuilder#AbstractStringBuilder(int)}
* 这里初始化一个长度为16的字节数组(char[])
* 调用链2：
* {@link java.lang.StringBuilder#append(String)}
* 这里调用父类的append方法，将当前字符串传给父类
* {@link java.lang.AbstractStringBuilder#append(String)}
* if (str == null)
* return appendNull();
* int len = str.length(); //这里length为1
* ensureCapacityInternal(count + len); //第一次append，count为0
* str.getChars(0, len, value, count);
* count += len; //count累加
* return this;
* {@link java.lang.AbstractStringBuilder#ensureCapacityInternal(int)}
* if (minimumCapacity - value.length > 0) { //这里minimumCapacity=1，value长度为16（即上面初始化时赋予的16个长度的容量）
* value = Arrays.copyOf(value, newCapacity(minimumCapacity));
* }
* {@link java.lang.String#getChars}
* if (srcBegin < 0) {
* throw new StringIndexOutOfBoundsException(srcBegin);
* }
* if (srcEnd > value.length) {
* throw new StringIndexOutOfBoundsException(srcEnd);
* }
* if (srcBegin > srcEnd) {
* throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
* }
* System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin); //调用native方法将append的字符串以char数组形式copy到value里
* count += len; //count重新赋值
* 调用链3：
* {@link java.lang.StringBuilder#append(int)}
* {@link java.lang.AbstractStringBuilder#append(int)}
* if (i == Integer.MIN_VALUE) {
* append("-2147483648");
* return this;
* }
* int appendedLength = (i < 0) ? Integer.stringSize(-i) + 1 : Integer.stringSize(i);
* int spaceNeeded = count + appendedLength;
* ensureCapacityInternal(spaceNeeded);
* Integer.getChars(i, spaceNeeded, value);
* count = spaceNeeded;
* return this;
* {@link java.lang.Integer#stringSize(int)}
* for (int i=0; ; i++)
* if (x <= sizeTable[i]) //sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999, 99999999, 999999999, Integer.MAX_VALUE }
* return i+1;
* {@link java.lang.AbstractStringBuilder#ensureCapacityInternal(int)}
* if (minimumCapacity - value.length > 0) {
* value = Arrays.copyOf(value, newCapacity(minimumCapacity));
* }
* {@link java.lang.Integer#getChars}
* count = count + appendedLength;
* 调用链4：
* {@link java.lang.StringBuilder#append(boolean)}
* {@link java.lang.AbstractStringBuilder#append(boolean)}
* {@link java.lang.AbstractStringBuilder#ensureCapacityInternal(int)}
* 调用链5：
* {@link java.lang.StringBuilder#append(char)}
* {@link java.lang.AbstractStringBuilder#append(char)}
* {@link java.lang.AbstractStringBuilder#ensureCapacityInternal(int)}
* 调用链6：
* {@link java.lang.StringBuilder#append(long)}
* {@link java.lang.AbstractStringBuilder#append(long)}
* if (l == Long.MIN_VALUE) {
* append("-9223372036854775808");
* return this;
* }
* int appendedLength = (l < 0) ? Long.stringSize(-l) + 1
* : Long.stringSize(l);
* int spaceNeeded = count + appendedLength;
* ensureCapacityInternal(spaceNeeded);
* Long.getChars(l, spaceNeeded, value);
* count = spaceNeeded;
* return this;
*/
@SuppressWarnings("all")
private static void createEmptyStringBuilder(){
//这里等同于上面的String str = a + b + c
StringBuilder sb = new StringBuilder(); //1
sb.append("a"); //2
sb.append("b");
sb.append(1); //3
sb.append(true); //4
sb.append('c'); //5
sb.append(99L); //6
System.out.println(sb.toString());
}
/**
* 创建包含初始容量的StringBuilder
* 调用链1：
* {@link java.lang.StringBuilder#StringBuilder(int)}
* {@link java.lang.AbstractStringBuilder#AbstractStringBuilder(int)}
* value = new char[capacity]; //capacity=3
* 调用链2：
* {@link java.lang.StringBuilder#append(String)}
* {@link java.lang.AbstractStringBuilder#append(String)}
* {@link java.lang.AbstractStringBuilder#ensureCapacityInternal(int)}
* if (minimumCapacity - value.length > 0) { //minimumCapacity=2, value.length=3
* value = Arrays.copyOf(value, newCapacity(minimumCapacity));
* }
* {@link java.lang.String#getChars}
* System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
* count += len;
* 调用链3：
* {@link java.lang.StringBuilder#append(String)}
* {@link java.lang.AbstractStringBuilder#append(String)}
* {@link java.lang.AbstractStringBuilder#ensureCapacityInternal(int)}
* if (minimumCapacity - value.length > 0) { //minimumCapacity=4, value.length=3
* value = Arrays.copyOf(value, newCapacity(minimumCapacity)); //value扩容
* }
* {@link java.util.Arrays#copyOf(float[], int)}
* float[] copy = new float[newLength];
* System.arraycopy(original, 0, copy, 0, Math.min(original.length, newLength));
* return copy;
* {@link java.lang.AbstractStringBuilder#newCapacity(int)}
* int newCapacity = (value.length << 1) + 2; //value.length=3, newCapacity=8
* if (newCapacity - minCapacity < 0) { //minCapacity=4
* newCapacity = minCapacity;
* }
* return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
* ? hugeCapacity(minCapacity) : newCapacity;
* {@link java.lang.String#getChars}
* System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
* count += len;
*/
@SuppressWarnings("all")
private static void createNonEmptyStringBuilder(){
StringBuilder sb = new StringBuilder(3); //1
sb.append("aa"); //2
sb.append("bb"); //3，触发扩容
System.out.println(sb.toString());
}
}
扩容方法
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
public static final int MAX_VALUE = 0x7fffffff;
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;
}
StringBuffer 扩容方式与 StringBuilder 基本一致
