一 String類
字符串存貯在一個final數組value中,並且未對外提供修改其內元素的方法,所以String類是不可變的。
private final char value[];
1.1 構造方法
1.2 length()和empty()
public String() { this.value = "".value; }
public String(String original) { this.value = original.value; this.hash = original.hash; }
通過上述構造方法可知,字符串的構造是通過將一個數組賦給當前String的value數組完成的。public String(char value[]) { this.value = Arrays.copyOf(value, value.length); }
public String(StringBuffer buffer) { synchronized(buffer) { this.value = Arrays.copyOf(buffer.getValue(), buffer.length()); } }
public String(StringBuilder builder) { this.value = Arrays.copyOf(builder.getValue(), builder.length()); }
由上述可知,用StringBuffer構造字符串是線程安全的,而StringBuilder是線程不安全的,通常應該優先使用StringBuilder類,因爲它支持所有與StringBuffer相同的操作,但由於它不執行同步,所以速度更快。
public int length() { return value.length; }
public boolean isEmpty() { return value.length == 0; }
length通過返回數組的長度取得。
1.3 charAt()
返回該索引對飲的數組值。public char charAt(int index) { if ((index < 0) || (index >= value.length)) { throw new StringIndexOutOfBoundsException(index); } return value[index]; }
1.5 indexOf()
通過返回ch在value中的所有取得。public int indexOf(int ch, int fromIndex) { final int max = value.length; if (fromIndex < 0) { fromIndex = 0; } else if (fromIndex >= max) { // Note: fromIndex might be near -1>>>1. return -1; } if (ch < Character.MIN_SUPPLEMENTARY_CODE_POINT) { // handle most cases here (ch is a BMP code point or a // negative value (invalid code point)) final char[] value = this.value; for (int i = fromIndex; i < max; i++) { if (value[i] == ch) { return i; } } return -1; } else { return indexOfSupplementary(ch, fromIndex); } }
1.6 subString()
若beginIndex==0則直接返回當前String,否則new一個新String。public String substring(int beginIndex) { if (beginIndex < 0) { throw new StringIndexOutOfBoundsException(beginIndex); } int subLen = value.length - beginIndex; if (subLen < 0) { throw new StringIndexOutOfBoundsException(subLen); } return (beginIndex == 0) ? this : new String(value, beginIndex, subLen); }
1.7 replace()
如果確實發生了字符替換,將會new 一個新的String返回。toUpperCase()等若發生改變也返回的是新的String。public String replace(char oldChar, char newChar) { if (oldChar != newChar) { int len = value.length; int i = -1; char[] val = value; /* avoid getfield opcode */ while (++i < len) { if (val[i] == oldChar) { break; } } if (i < len) { char buf[] = new char[len]; for (int j = 0; j < i; j++) { buf[j] = val[j]; } while (i < len) { char c = val[i]; buf[i] = (c == oldChar) ? newChar : c; i++; } return new String(buf, true); } } return this; }
1.8 intern
public native String intern();
如果池已經包含一個等於此 String 對象的字符串(該對象由 equals(Object) 方法確定),則返回池中的字符串。否則,將此 String 對象添加到池中,並且返回此 String 對象的引用。
二 StringBuffer類
2.1 構造函數
其父類構造函數爲:public StringBuffer() { super(16); }
即默認構造了大小爲16的數組。AbstractStringBuilder(int capacity) { value = new char[capacity]; }
構造一個比當前字符串長16的字符串。public StringBuffer(String str) { super(str.length() + 16); append(str); }
2.2 append()
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; }
private void ensureCapacityInternal(int minimumCapacity) { // overflow-conscious code if (minimumCapacity - value.length > 0) expandCapacity(minimumCapacity); }
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); }
每次擴容爲原數組長度的2倍+2,若不夠則取count+len,即每次擴容胡至少爲minimumCapacity=count+len。
三 StringBuilder類
3.1 構造函數
public StringBuilder() { super(16); }
類似StringBuffer。
四 StringBuffer VS StringBuilder
StringBuffer類的成員方法前面多了一個關鍵字:synchronized,所以它是線程安全的,由此帶來的問題是效率低。