1. Public String subString(int beginIndex)
String的共有方法，从beginIndex位置开始截取字符串到源字符串末尾,包括beginIndex
"123".subString(1)--->"23"
/**返回目标String中截取的指定String * Returns a string that is a substring of this string. The * substring begins with the character at the specified index and * extends to the end of this string. <p> * Examples: * <blockquote><pre> * "unhappy".substring(2) returns "happy" * "Harbison".substring(3) returns "bison" * "emptiness".substring(9) returns "" (an empty string) * </pre></blockquote> * * @param beginIndex the beginning index, inclusive. 开始位(包括在结果中) * @return the specified substring. 指定的Strng * @exception IndexOutOfBoundsException if * {@code beginIndex} is negative or larger than the * length of this {@code String} object. */ public String substring(int beginIndex) { if (beginIndex < 0) { //当开始位beginIndex小于0抛出异常StringIndexOutOfBoundsException() throw new StringIndexOutOfBoundsException(beginIndex); } int subLen = value.length - beginIndex; if (subLen < 0) { //要截取的String的长度小于0时抛出异常StringIndexOutOfBoundsException() throw new StringIndexOutOfBoundsException(subLen); } return (beginIndex == 0) ? this : new String(value, beginIndex, subLen); //当开始位beginIndex等于0时相当于返回字符串本身否则返回新的String }
2.public String substring(int beginIndex, int endIndex)
String的共有方法，从beginIndex位置开始截取字符串到endIndex,包括beginIndex,但是不包括endIndex
"123".subString(1,2)--->"2"
/** * Returns a string that is a substring of this string. The * substring begins at the specified {@code beginIndex} and * extends to the character at index {@code endIndex - 1}. * Thus the length of the substring is {@code endIndex-beginIndex}. * <p>
* 返回一个从本字符串截取的新的字符串 * Examples: * <blockquote><pre> * "hamburger".substring(4, 8) returns "urge" * "smiles".substring(1, 5) returns "mile" * </pre></blockquote> * * @param beginIndex the beginning index, inclusive. 开始截取位置（包括在内） * @param endIndex the ending index, exclusive. 结束位置（不包括在内） * @return the specified substring. * @exception IndexOutOfBoundsException if the * {@code beginIndex} is negative, or * {@code endIndex} is larger than the length of * this {@code String} object, or * {@code beginIndex} is larger than * {@code endIndex}. */ public String substring(int beginIndex, int endIndex) { if (beginIndex < 0) { //开始位置小于0时抛出异常 throw new StringIndexOutOfBoundsException(beginIndex); } if (endIndex > value.length) { //结束位置大于本字符串长度时抛出异常（String通过char[] value储存） throw new StringIndexOutOfBoundsException(endIndex); } int subLen = endIndex - beginIndex; if (subLen < 0) { //截取长度小于0时抛出异常 throw new StringIndexOutOfBoundsException(subLen); } return ((beginIndex == 0) && (endIndex == value.length)) ? this //当开始位置等与0且结束位置等于字符串长度时返回本身，否则新建一个String返回 : new String(value, beginIndex, subLen); }
3.public String(char value[],int offset,int count)
String的构造方法，根据char[]value数组作为源数组，新建一个String，从第offset位置开始，一共截取count个字符，结果包括offset
String test=new String(new char[]{'1','2','3'},1,2);
test------>23
/** * Allocates a new {@code String} that contains characters from a subarray * of the character array argument. The {@code offset} argument is the * index of the first character of the subarray and the {@code count} * argument specifies the length of the subarray. The contents of the * subarray are copied; subsequent modification of the character array does * not affect the newly created string.
* 分配一个新的String，从一个char[]数组截取（String类带有char[] value成员，它是String的主要存储结构），
* 参数offset是开始复制的位置，count是复制的长度大小 * * @param value * Array that is the source of characters 源数组 * * @param offset * The initial offset 初始位 * * @param count * The length 复制的长度 * * @throws IndexOutOfBoundsException * If the {@code offset} and {@code count} arguments index * characters outside the bounds of the {@code value} array */ public String(char value[], int offset, int count) { if (offset < 0) { //开始位小于0抛出异常 throw new StringIndexOutOfBoundsException(offset); } if (count <= 0) { //复制长度小于等于0时 if (count < 0) { //复制长度小于0抛出异常 throw new StringIndexOutOfBoundsException(count); } if (offset <= value.length) { //复制长度等于0且开始位置小于等于value长度时返回空的String this.value = "".value; return; } } // Note: offset or count might be near -1>>>1. if (offset > value.length - count) { //开始位+长度大于源char数组的长度时抛出错误 throw new StringIndexOutOfBoundsException(offset + count); } this.value = Arrays.copyOfRange(value, offset, offset+count); //调用Arrays.copyRange进行复制 }
4.public static char[] copyOfRange(char[] original, int from, int to)
Arrays的共有方法，按范围复制并返回一个新的char[]数组,从from位置开始（包括from），到to位置结束（不包括to）
char[] test=Array.copyOfRange(new char[]{'1','2','3'},1,2);
test------>23
/** * Copies the specified range of the specified array into a new array. * The initial index of the range (<tt>from</tt>) must lie between zero * and <tt>original.length</tt>, inclusive. The value at * <tt>original[from]</tt> is placed into the initial element of the copy * (unless <tt>from == original.length</tt> or <tt>from == to</tt>). * Values from subsequent elements in the original array are placed into * subsequent elements in the copy. The final index of the range * (<tt>to</tt>), which must be greater than or equal to <tt>from</tt>, * may be greater than <tt>original.length</tt>, in which case * <tt>'\u000'</tt> is placed in all elements of the copy whose index is * greater than or equal to <tt>original.length - from</tt>. The length * of the returned array will be <tt>to - from</tt>. * 从源char[]数组复制成新的char[]数组
* * @param original the array from which a range is to be copied 源char[]数组 * @param from the initial index of the range to be copied, inclusive 源数组的开始复制位置(包括在内) * @param to the final index of the range to be copied, exclusive. 源数组的复制结束位置（不包括在内） * (This index may lie outside the array.) * @return a new array containing the specified range from the original array, * truncated or padded with null characters to obtain the required length * @throws ArrayIndexOutOfBoundsException if {@code from < 0} * or {@code from > original.length} * @throws IllegalArgumentException if <tt>from > to</tt> * @throws NullPointerException if <tt>original</tt> is null * @since 1.6 */ public static char[] copyOfRange(char[] original, int from, int to) { int newLength = to - from; if (newLength < 0) //新数组长度小于0抛出异常 throw new IllegalArgumentException(from + " > " + to); char[] copy = new char[newLength]; //新建一个数组 System.arraycopy(original, from, copy, 0, //调用arraycopy来复制修改新数组的值 Math.min(original.length - from, newLength)); return copy; }