JDK1.8源码阅读（三）Java.lang.AbstractStringBuilder

最新推荐文章于 2024-05-13 01:00:00 发布

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最新推荐文章于 2024-05-13 01:00:00 发布

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AbstractStringBuilder是Java中的一个关键类，作为StringBuilder和StringBuffer的基础，提供了一系列用于字符串操作的方法，包括拼接、插入、删除等。它实现了Appendable接口和CharSequence接口，支持高效地构建和修改字符串。

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AbstractStringBuilder定义

abstract class AbstractStringBuilder implements Appendable, CharSequence {}

AbstractStringBuilder是一个抽象类，实现了Appendable接口，能够被添加char序列和值，还实现了CharSequence可读的字符序列。

AbstractStringBuilder属性

char[] value;//用来存储字符串，跟String不同在于没有final修饰。
int count;//字符串长度

AbstractStringBuilder构造方法

AbstractStringBuilder方法

1.public int length()

返回以存储长度

2.public int capacity()

返回数组当前容量

/*保证容量至少等于指定最小值*/
public void ensureCapacity(int minimumCapacity) {
        //如果参数大于0
        if (minimumCapacity > 0)
            //确认内部容量
            ensureCapacityInternal(minimumCapacity);
    }

private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        //如果参数大于数组长度
        if (minimumCapacity - value.length > 0) {
           //数组扩容
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }

/*扩容方法*/
private int newCapacity(int minCapacity) {
        // overflow-conscious code
        //扩容后的容量，扩大一倍再+2
        int newCapacity = (value.length << 1) + 2;
        //如果扩容后的长度比参数小，那么扩容至参数长度=传入参数。保证至少等于参数的长度
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        //newCapacity>minCapacity>MAX_ARRAY_SIZE，
        //会扩容至minCapacity，如果此时minCapacity<MAX_ARRAY_SIZE
        //会扩容至MAX_ARRAY_SIZE
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }

private int hugeCapacity(int minCapacity) {
        //如果参数超出int的最大值，抛出异常
        if (Integer.MAX_VALUE - minCapacity < 0) { // overflow
            throw new OutOfMemoryError();
        }
        //如果参数大于最大值，那么返回最大值，否则返回最大值
        return (minCapacity > MAX_ARRAY_SIZE)
            ? minCapacity : MAX_ARRAY_SIZE;
    }

//去除多余数组容量
public void trimToSize() {
        //如果字符串长度<数组长度，那么数组长度等于字符串长度
        if (count < value.length) {
            value = Arrays.copyOf(value, count);
        }
    }

/**
	* 设置数组实际使用的长度，如果newLength>ccount，对其进行扩容，
	* 将扩容的部分使用 '\0' (空字符)代替。
	*/
public void setLength(int newLength) {
        if (newLength < 0)
            throw new StringIndexOutOfBoundsException(newLength);
        ensureCapacityInternal(newLength);

        if (count < newLength) {
            Arrays.fill(value, count, newLength, '\0');
        }

        count = newLength;
    }

9.append

//传入对象，append为String
public AbstractStringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }
//将字符串以char[]返回
final char[] getValue() {
        return value;
    }
//appendString对象
public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }
//appendStringbuffer
public AbstractStringBuilder append(StringBuffer sb) {
        if (sb == null)
            return appendNull();
        int len = sb.length();
        ensureCapacityInternal(count + len);
        sb.getChars(0, len, value, count);
        count += len;
        return this;
    }
//1.8新增append方法
AbstractStringBuilder append(AbstractStringBuilder asb) {
        if (asb == null)
            return appendNull();
        int len = asb.length();
        ensureCapacityInternal(count + len);
        asb.getChars(0, len, value, count);
        count += len;
        return this;
    }
//appendCharSequence
public AbstractStringBuilder append(CharSequence s) {
        if (s == null)
            return appendNull();
        if (s instanceof String)
            return this.append((String)s);
        if (s instanceof AbstractStringBuilder)
            return this.append((AbstractStringBuilder)s);

        return this.append(s, 0, s.length());
    }
//append Null方法
private AbstractStringBuilder appendNull() {
        int c = count;
        ensureCapacityInternal(c + 4);
        final char[] value = this.value;
        value[c++] = 'n';
        value[c++] = 'u';
        value[c++] = 'l';
        value[c++] = 'l';
        count = c;
        return this;
    }
//指定位置append
public AbstractStringBuilder append(CharSequence s, int start, int end) {
        if (s == null)
            s = "null";
        if ((start < 0) || (start > end) || (end > s.length()))
            throw new IndexOutOfBoundsException(
                "start " + start + ", end " + end + ", s.length() "
                + s.length());
        int len = end - start;
        ensureCapacityInternal(count + len);
        for (int i = start, j = count; i < end; i++, j++)
            value[j] = s.charAt(i);
        count += len;
        return this;
    }
//append 数组
public AbstractStringBuilder append(char[] str) {
        int len = str.length;
        ensureCapacityInternal(count + len);
        System.arraycopy(str, 0, value, count, len);
        count += len;
        return this;
    }
//append 数组指定几个字符
public AbstractStringBuilder append(char str[], int offset, int len) {
        if (len > 0)                // let arraycopy report AIOOBE for len < 0
            ensureCapacityInternal(count + len);
        System.arraycopy(str, offset, value, count, len);
        count += len;
        return this;
    }
//append Boolean
public AbstractStringBuilder append(boolean b) {
        if (b) {
            ensureCapacityInternal(count + 4);
            value[count++] = 't';
            value[count++] = 'r';
            value[count++] = 'u';
            value[count++] = 'e';
        } else {
            ensureCapacityInternal(count + 5);
            value[count++] = 'f';
            value[count++] = 'a';
            value[count++] = 'l';
            value[count++] = 's';
            value[count++] = 'e';
        }
        return this;
    }
//append Char
public AbstractStringBuilder append(char c) {
        ensureCapacityInternal(count + 1);
        value[count++] = c;
        return this;
    }
//append int 
public AbstractStringBuilder append(int i) {
        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;
    }
//long 
public AbstractStringBuilder append(long l) {
        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;
    }
//float
public AbstractStringBuilder append(float f) {
        FloatingDecimal.appendTo(f,this);
        return this;
    }
//dobble
public AbstractStringBuilder append(double d) {
        FloatingDecimal.appendTo(d,this);
        return this;
    }
//根据索引删除指定字符
public AbstractStringBuilder delete(int start, int end) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (end > count)
            end = count;
        if (start > end)
            throw new StringIndexOutOfBoundsException();
        int len = end - start;
        if (len > 0) {
            System.arraycopy(value, start+len, value, start, count-end);
            count -= len;
        }
        return this;
    }
//追加码点
public AbstractStringBuilder appendCodePoint(int codePoint) {
        final int count = this.count;

        if (Character.isBmpCodePoint(codePoint)) {
            ensureCapacityInternal(count + 1);
            value[count] = (char) codePoint;
            this.count = count + 1;
        } else if (Character.isValidCodePoint(codePoint)) {
            ensureCapacityInternal(count + 2);
            Character.toSurrogates(codePoint, value, count);
            this.count = count + 2;
        } else {
            throw new IllegalArgumentException();
        }
        return this;
    }
//删除索引的字符
public AbstractStringBuilder deleteCharAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        System.arraycopy(value, index+1, value, index, count-index-1);
        count--;
        return this;
    }
//指定索引替换
public AbstractStringBuilder replace(int start, int end, String str) {
        if (start < 0)
            throw new StringIndexOutOfBoundsException(start);
        if (start > count)
            throw new StringIndexOutOfBoundsException("start > length()");
        if (start > end)
            throw new StringIndexOutOfBoundsException("start > end");

        if (end > count)
            end = count;
        int len = str.length();
        int newCount = count + len - (end - start);
        ensureCapacityInternal(newCount);

        System.arraycopy(value, end, value, start + len, count - end);
        str.getChars(value, start);
        count = newCount;
        return this;
    }
//
public String substring(int start) {
        return substring(start, count);
    }

//将数组，根据指定索引添加到字符串
public AbstractStringBuilder insert(int index, char[] str, int offset,
                                        int len)
    {
        if ((index < 0) || (index > length()))
            throw new StringIndexOutOfBoundsException(index);
        if ((offset < 0) || (len < 0) || (offset > str.length - len))
            throw new StringIndexOutOfBoundsException(
                "offset " + offset + ", len " + len + ", str.length "
                + str.length);
        ensureCapacityInternal(count + len);
        System.arraycopy(value, index, value, index + len, count - index);
        System.arraycopy(str, offset, value, index, len);
        count += len;
        return this;
    }
//传入对象，以字符串形式在指定位置添加字符
public AbstractStringBuilder insert(int offset, Object obj) {
        return insert(offset, String.valueOf(obj));
    }
//传入字符串在指定位置添加
public AbstractStringBuilder insert(int offset, String str) {
        if ((offset < 0) || (offset > length()))
            throw new StringIndexOutOfBoundsException(offset);
        if (str == null)
            str = "null";
        int len = str.length();
        ensureCapacityInternal(count + len);
        System.arraycopy(value, offset, value, offset + len, count - offset);
        str.getChars(value, offset);
        count += len;
        return this;
    }
//在指定位置添加数组
public AbstractStringBuilder insert(int offset, char[] str) {
        if ((offset < 0) || (offset > length()))
            throw new StringIndexOutOfBoundsException(offset);
        int len = str.length;
        ensureCapacityInternal(count + len);
        System.arraycopy(value, offset, value, offset + len, count - offset);
        System.arraycopy(str, 0, value, offset, len);
        count += len;
        return this;
    }
//指定位置添加CharSequence
public AbstractStringBuilder insert(int dstOffset, CharSequence s) {
        if (s == null)
            s = "null";
        if (s instanceof String)
            return this.insert(dstOffset, (String)s);
        return this.insert(dstOffset, s, 0, s.length());
    }
 public AbstractStringBuilder insert(int dstOffset, CharSequence s,
                                         int start, int end) {
        if (s == null)
            s = "null";
        if ((dstOffset < 0) || (dstOffset > this.length()))
            throw new IndexOutOfBoundsException("dstOffset "+dstOffset);
        if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))
            throw new IndexOutOfBoundsException(
                "start " + start + ", end " + end + ", s.length() "
                + s.length());
        int len = end - start;
        ensureCapacityInternal(count + len);
        System.arraycopy(value, dstOffset, value, dstOffset + len,
                         count - dstOffset);
        for (int i=start; i<end; i++)
            value[dstOffset++] = s.charAt(i);
        count += len;
        return this;
    }
//添加Boolean
public AbstractStringBuilder insert(int offset, boolean b) {
        return insert(offset, String.valueOf(b));
    }
//添加Char
public AbstractStringBuilder insert(int offset, char c) {
        ensureCapacityInternal(count + 1);
        System.arraycopy(value, offset, value, offset + 1, count - offset);
        value[offset] = c;
        count += 1;
        return this;
    }
//添加int
public AbstractStringBuilder insert(int offset, int i) {
        return insert(offset, String.valueOf(i));
    }
//添加long
public AbstractStringBuilder insert(int offset, long l) {
        return insert(offset, String.valueOf(l));
    }
//folat
public AbstractStringBuilder insert(int offset, float f) {
        return insert(offset, String.valueOf(f));
    }
//doble
public AbstractStringBuilder insert(int offset, double d) {
        return insert(offset, String.valueOf(d));
    }
//String
public int indexOf(String str) {
        return indexOf(str, 0);
    }
//
public int indexOf(String str, int fromIndex) {
        return String.indexOf(value, 0, count, str, fromIndex);
    }
//返回传入字符在字符串最后出现的下标
public int lastIndexOf(String str) {
        return lastIndexOf(str, count);
    }
public int lastIndexOf(String str, int fromIndex) {
        return String.lastIndexOf(value, 0, count, str, fromIndex);
    }
//
/**
	 * AbstractStringBuilder的value进行反转,
	 * 如果有字符在代理单元中，进行reverseAllValidSurrogatePairs操作
	 */
	public AbstractStringBuilder reverse() {
        boolean hasSurrogates = false;
        int n = count - 1;
        for (int j = (n-1) >> 1; j >= 0; j--) {
            int k = n - j;
            char cj = value[j];
            char ck = value[k];
            value[j] = ck;
            value[k] = cj;
            if (Character.isSurrogate(cj) ||
                Character.isSurrogate(ck)) {
                hasSurrogates = true;
            }
        }
        if (hasSurrogates) {
            reverseAllValidSurrogatePairs();
        }
        return this;
    }

	//有时间研究下Unicode，更好de理解这块
	/** Outlined helper method for reverse() */
	private void reverseAllValidSurrogatePairs() {
        for (int i = 0; i < count - 1; i++) {
            char c2 = value[i];
            if (Character.isLowSurrogate(c2)) {
                char c1 = value[i + 1];
                if (Character.isHighSurrogate(c1)) {
                    value[i++] = c1;
                    value[i] = c2;
                }
            }
        }
    }

StringBuilder与StringBuffer的方法基本都是调用此类的实现，只是StringBuffer的方法使用synchronized['sɪŋkrənaɪzd]来修饰，是线程安全的，所以相比与StringBuilder，效率会低。