介绍
String 类是 final 修饰，说明不可以被继承，String 一旦被创建就不能改变，对 String 修改就会创建一个新的字符串，因此禁止使用循环修改字符串。
Sring 在内存中的位置
String str1 = new String("hello"); String str2 = new String("hello"); String str3 = "hello"; String str4 = "hello"; String str5 = "he"+"llo"; String str6 = "he"; String str7 = "llo"; System.out.println(str1==str2); //false System.out.println(str1==str3); //false System.out.println(str3==str4); //true System.out.println(str4 == str5); //true System.out.println(str3=="hello"); //true System.out.println(str4==(str6+str7)); //false
使用 new 创建一个 String 对象，会在堆内分配内存，每个对象地址都不一样，而 str3 会先在字符串常量池查找，没有才会添加进去，如果存在直接引用地址。最后 str6+str7 是在 JVM 运行时才操作，会创建一个新的对象。
源码
继承
public final class String implements java.io.Serializable, Comparable<String>, CharSequence {}
String 实现了 Serializable、Comparable 和 CharSequence 接口
成员变量
//存放final类型的char数组，不可修改 private final char value[]; //hash值 private int hash; private static final ObjectStreamField[] serialPersistentFields = new ObjectStreamField[0];
构造函数
//创建一个空的字符串 public String() { this.value = "".value; } //新创建的String是参数String的副本 public String(String original) { this.value = original.value; this.hash = original.hash; } //传入char数组，直接拷贝数据到value public String(char value[]) { this.value = Arrays.copyOf(value, value.length); } //offset是第一个字符的索引，count是字节数，表示char数组从offset开始，获取长度为count的子数组 public String(char value[], int offset, int count) { if (offset < 0) { throw new StringIndexOutOfBoundsException(offset); } if (count <= 0) { if (count < 0) { throw new StringIndexOutOfBoundsException(count); } //长度为0 if (offset <= value.length) { this.value = "".value; return; } } // 数组越界 if (offset > value.length - count) { throw new StringIndexOutOfBoundsException(offset + count); } this.value = Arrays.copyOfRange(value, offset, offset+count); } //Unicode 代码点数组参数一个子数组的字符 public String(int[] codePoints, int offset, int count) { if (offset < 0) { throw new StringIndexOutOfBoundsException(offset); } if (count <= 0) { if (count < 0) { throw new StringIndexOutOfBoundsException(count); } if (offset <= codePoints.length) { this.value = "".value; return; } } // Note: offset or count might be near -1>>>1. if (offset > codePoints.length - count) { throw new StringIndexOutOfBoundsException(offset + count); } final int end = offset + count; // Pass 1: Compute precise size of char[] int n = count; for (int i = offset; i < end; i++) { int c = codePoints[i]; if (Character.isBmpCodePoint(c)) continue; else if (Character.isValidCodePoint(c)) n++; else throw new IllegalArgumentException(Integer.toString(c)); } // Pass 2: Allocate and fill in char[] final char[] v = new char[n]; for (int i = offset, j = 0; i < end; i++, j++) { int c = codePoints[i]; if (Character.isBmpCodePoint(c)) v[j] = (char)c; else Character.toSurrogates(c, v, j++); } this.value = v; } //不推荐使用 @Deprecated public String(byte ascii[], int hibyte, int offset, int count) { checkBounds(ascii, offset, count); char value[] = new char[count]; if (hibyte == 0) { for (int i = count; i-- > 0;) { value[i] = (char)(ascii[i + offset] & 0xff); } } else { hibyte <<= 8; for (int i = count; i-- > 0;) { value[i] = (char)(hibyte | (ascii[i + offset] & 0xff)); } } this.value = value; } //不推荐使用 @Deprecated public String(byte ascii[], int hibyte) { this(ascii, hibyte, 0, ascii.length); } // public String(byte bytes[], int offset, int length, String charsetName) throws UnsupportedEncodingException { if (charsetName == null) throw new NullPointerException("charsetName"); checkBounds(bytes, offset, length); this.value = StringCoding.decode(charsetName, bytes, offset, length); } //传入字节数组，使用指定字符集 public String(byte bytes[], int offset, int length, Charset charset) { if (charset == null) throw new NullPointerException("charset"); checkBounds(bytes, offset, length); this.value = StringCoding.decode(charset, bytes, offset, length); } public String(byte bytes[], String charsetName) throws UnsupportedEncodingException { this(bytes, 0, bytes.length, charsetName); } public String(byte bytes[], Charset charset) { this(bytes, 0, bytes.length, charset); } public String(byte bytes[], int offset, int length) { checkBounds(bytes, offset, length); this.value = StringCoding.decode(bytes, offset, length); } public String(byte bytes[]) { this(bytes, 0, bytes.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()); } String(char[] value, boolean share) { // assert share : "unshared not supported"; this.value = value; }
其他方法
//获取长度 public int length() { return value.length; } //判断是否为空 public boolean isEmpty() { return value.length == 0; } //获取char指定索引的值 public char charAt(int index) { if ((index < 0) || (index >= value.length)) { throw new StringIndexOutOfBoundsException(index); } return value[index]; } //获取指定索引处的字符（Unicode代码点） public int codePointAt(int index) { if ((index < 0) || (index >= value.length)) { throw new StringIndexOutOfBoundsException(index); } return Character.codePointAtImpl(value, index, value.length); } //获取指定索引之前的字符 public int codePointBefore(int index) { int i = index - 1; if ((i < 0) || (i >= value.length)) { throw new StringIndexOutOfBoundsException(index); } return Character.codePointBeforeImpl(value, index, 0); } //获取String文本范围内Unicode代码点数 public int codePointCount(int beginIndex, int endIndex) { if (beginIndex < 0 || endIndex > value.length || beginIndex > endIndex) { throw new IndexOutOfBoundsException(); } return Character.codePointCountImpl(value, beginIndex, endIndex - beginIndex); } //返回此 String 中从给定的 index 处偏移 codePointOffset 个代码点的索引。 public int offsetByCodePoints(int index, int codePointOffset) { if (index < 0 || index > value.length) { throw new IndexOutOfBoundsException(); } return Character.offsetByCodePointsImpl(value, 0, value.length, index, codePointOffset); } //将字符串中的字符复制到目标数组中，dstBegin是目标数组中的起始偏移量 void getChars(char dst[], int dstBegin) { System.arraycopy(value, 0, dst, dstBegin, value.length); } //同上，要复制字符串中，从srcBegin开始，到srcEnd-1结束 public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) { 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); } //获取命名的字符集字节数组 public byte[] getBytes(String charsetName) throws UnsupportedEncodingException { if (charsetName == null) throw new NullPointerException(); return StringCoding.encode(charsetName, value, 0, value.length); } //获取指定字符集的字节数组 public byte[] getBytes(Charset charset) { if (charset == null) throw new NullPointerException(); return StringCoding.encode(charset, value, 0, value.length); } //获取默认字符集的字节数组 public byte[] getBytes() { return StringCoding.encode(value, 0, value.length); } //重写equals方法 public boolean equals(Object anObject) { //如果对象地址相同，就是相同的对象 if (this == anObject) { return true; } //首先判断是否是String对象 if (anObject instanceof String) { String anotherString = (String)anObject; int n = value.length; //判断长度是否相同，不相同不是相同的字符串 if (n == anotherString.value.length) { char v1[] = value; char v2[] = anotherString.value; int i = 0; //判断每一个字符是否相同 while (n-- != 0) { if (v1[i] != v2[i]) return false; i++; } return true; } } return false; } //equals对比的是String对象，contentEquals对比CharSequence或其子类的对象 public boolean contentEquals(StringBuffer sb) { return contentEquals((CharSequence)sb); } public boolean contentEquals(CharSequence cs) { // Argument is a StringBuffer, StringBuilder if (cs instanceof AbstractStringBuilder) { if (cs instanceof StringBuffer) { //StringBuffer 是线程安全的，用synchronized块 synchronized(cs) { return nonSyncContentEquals((AbstractStringBuilder)cs); } } else { return nonSyncContentEquals((AbstractStringBuilder)cs); } } // 如果是String，就用equals方法 if (cs instanceof String) { return equals(cs); } // Argument is a generic CharSequence char v1[] = value; int n = v1.length; if (n != cs.length()) { return false; } for (int i = 0; i < n; i++) { if (v1[i] != cs.charAt(i)) { return false; } } return true; } private boolean nonSyncContentEquals(AbstractStringBuilder sb) { char v1[] = value; char v2[] = sb.getValue(); int n = v1.length; //判断长度是否相同 if (n != sb.length()) { return false; } //对比每一个字符 for (int i = 0; i < n; i++) { if (v1[i] != v2[i]) { return false; } } return true; } //对比String，忽略大小写 public boolean equalsIgnoreCase(String anotherString) { return (this == anotherString) ? true : (anotherString != null) && (anotherString.value.length == value.length) && regionMatches(true, 0, anotherString, 0, value.length); } //如果字符串等于参数字符串anotherString，返回0；如果字符串小于参数字符串anotherString，返回小于0的值；如果字符串大于参数字符串anotherString，返回大于0的值 public int compareTo(String anotherString) { int len1 = value.length; int len2 = anotherString.value.length; int lim = Math.min(len1, len2);//取长度最小的 char v1[] = value; char v2[] = anotherString.value; int k = 0; while (k < lim) { char c1 = v1[k]; char c2 = v2[k]; //如果不相同，就比较不相同的字符 if (c1 != c2) { return c1 - c2; } k++; } //上面遍历结束还未返回，如果两个长度相同那说明两个字符串完全相同，返回0 return len1 - len2; } //忽略大小写的对比 public static final Comparator<String> CASE_INSENSITIVE_ORDER = new CaseInsensitiveComparator(); private static class CaseInsensitiveComparator implements Comparator<String>, java.io.Serializable { // use serialVersionUID from JDK 1.2.2 for interoperability private static final long serialVersionUID = 8575799808933029326L; public int compare(String s1, String s2) { int n1 = s1.length(); int n2 = s2.length(); int min = Math.min(n1, n2); for (int i = 0; i < min; i++) { char c1 = s1.charAt(i); char c2 = s2.charAt(i); if (c1 != c2) { c1 = Character.toUpperCase(c1); c2 = Character.toUpperCase(c2); if (c1 != c2) { c1 = Character.toLowerCase(c1); c2 = Character.toLowerCase(c2); if (c1 != c2) { // No overflow because of numeric promotion return c1 - c2; } } } } return n1 - n2; } private Object readResolve() { return CASE_INSENSITIVE_ORDER; } } //忽略大小写 public int compareToIgnoreCase(String str) { return CASE_INSENSITIVE_ORDER.compare(this, str); } public int indexOf(int ch) { return indexOf(ch, 0); } //返回指定字符第一次出现的字符串内的索引，从fromIndex开始查找，ch是字符，Unicode代码点 public int indexOf(int ch, int fromIndex) { final int max = value.length; //如果偏移量小于0，则从0开始查 if (fromIndex < 0) { fromIndex = 0; } else if (fromIndex >= max) { // 如果偏移量大于字符数量，那就返回-1，表示没找到 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; //循环查找，只返回第一次出现的位置 for (int i = fromIndex; i < max; i++) { if (value[i] == ch) { return i; } } return -1; } else { return indexOfSupplementary(ch, fromIndex); } } //增补字符验证 private int indexOfSupplementary(int ch, int fromIndex) { if (Character.isValidCodePoint(ch)) { final char[] value = this.value; final char hi = Character.highSurrogate(ch); final char lo = Character.lowSurrogate(ch); final int max = value.length - 1; for (int i = fromIndex; i < max; i++) { if (value[i] == hi && value[i + 1] == lo) { return i; } } } return -1; } //返回指定字符最后一次出现的位置 public int lastIndexOf(int ch) { return lastIndexOf(ch, value.length - 1); } public int lastIndexOf(int ch, int fromIndex) { 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; int i = Math.min(fromIndex, value.length - 1); for (; i >= 0; i--) { if (value[i] == ch) { return i; } } return -1; } else { return lastIndexOfSupplementary(ch, fromIndex); } } private int lastIndexOfSupplementary(int ch, int fromIndex) { if (Character.isValidCodePoint(ch)) { final char[] value = this.value; char hi = Character.highSurrogate(ch); char lo = Character.lowSurrogate(ch); int i = Math.min(fromIndex, value.length - 2); for (; i >= 0; i--) { if (value[i] == hi && value[i + 1] == lo) { return i; } } } return -1; } public int indexOf(String str) { return indexOf(str, 0); } public int indexOf(String str, int fromIndex) { return indexOf(value, 0, value.length, str.value, 0, str.value.length, fromIndex); } //去除字符串两端空格 public String trim() { int len = value.length; int st = 0; char[] val = value; /* avoid getfield opcode */ while ((st < len) && (val[st] <= ' ')) { st++; } while ((st < len) && (val[len - 1] <= ' ')) { len--; } //判断前后空格，然后取子集 return ((st > 0) || (len < value.length)) ? substring(st, len) : this; }
其他的不看了。。
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