Java字符串知识简单汇总
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Java.String
String 类的两种赋值方式
- String 可以表示出一个字符串,根据String的源码我们会发现String类实际上是使用字符数组char[]存储的.
public final class String implements java.io.Serializable, Comparable<String>, CharSequence { /** The value is used for character storage. */ private final char value[]; /** Cache the hash code for the string */ private int hash; // Default to 0 /** use serialVersionUID from JDK 1.0.2 for interoperability */ private static final long serialVersionUID = -6849794470754667710L; public String() { this.value = "".value; } public String(String original) { this.value = original.value; this.hash = original.hash; } public String(char value[]) { this.value = Arrays.copyOf(value, value.length); } 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); } if (offset <= value.length) { this.value = "".value; return; } } // Note: offset or count might be near -1>>>1. if (offset > value.length - count) { throw new StringIndexOutOfBoundsException(offset + count); } this.value = Arrays.copyOfRange(value, offset, offset+count); } 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; } 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); } 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()); }常见的两种String赋值方式
- String str1 = new String(“name”);
- String str2 = “name”;
两种赋值方式分析
//会创建两个字符串对象，一个在常量池中创建便于下一次使用，一个在堆内存中创建String str1 = new String("name");//最多创建一个字符串对象，有可能不用创建对象（如果常量池中已经存在相应的常量）String str2 = "name";//推荐使用内存分析
String类编译期与运行期分析
- 首先明确对象之间 “==” 是比较两个对象的地址。
public class StringDemo { public static void main(String[] args) { // 情况一 结果为true String aString = "a1"; String a1String = "a" + 1; System.out.println(aString == a1String); // 情况二 结果为false String bString = "b1"; int bb = 1; String b1String = "b" + bb; System.out.println(bString == b1String); // 情况三 结果为true String cString = "c1"; final int cc = 1; String c1String = "c" + cc; System.out.println(cString == c1String); // 情况四 结果为false String dString = "dd"; final int dd = getDD(); String d1String = dString + dd; System.out.println(dString == d1String); } public static int getDD(){ return 1; }}- 情况二为false，因为 b1String 在编译期时是还没有完全确定下来的，因为 bb 是一个变量，只有在执行期才能加载到。
- 情况四为false，注意与情况三进行对比，虽然使用了final关键字，但是根据函数返回值确定下来的，而函数要执行仍然是在运行期的时候确定的，编译期仍然无法确定。
String 类字符与字符串操作方法
- 字符与字符串操作
- 字节与字符串操作
- 是否以指定内容开头或结尾
- String 类替换操作
- String 类字符串截取操作
- 字符串拆分操作
- String 类字符串查找操作
- String 类其他操作方法
更多请见相关api文档
常见用例
System.out.println("abcde".charAt(3));//dSystem.out.println("abcde".toCharArray()[2]);//cbyte[] bytes = "abcde".getBytes();for(int i = 0; i < bytes.length; i++){ System.out.println(bytes[i]);}System.out.println(new String(bytes).toString());System.out.println(new String(bytes,0,2).toString());//abSystem.out.println(new String(bytes,"utf-8").toString());System.out.println("abcde".startsWith("ab"));//trueSystem.out.println("abcde".startsWith("cd",2));//trueSystem.out.println("abcde".endsWith("de"));//trueSystem.out.println("abc,de".replace(",", ":").toString());//abc:deSystem.out.println("abcde".replace("ab", "AB"));//ABcde//匹配正则表达式中26个字母替换为相应的字符串System.out.println("abcde".replaceAll("[a-z]", "*"));//*****System.out.println("abcde".replaceFirst("[a-z]", "*"));//*bcde//包括起始位置System.out.println("abcde".substring(2));//cde//包括起始位置，不包括结束位置System.out.println("abcde".substring(1, 3));//bc String[] value = "ab-cde".split("-");//传入的参数为正则表达式String[] value2 = "ab-c-de".split("-",3);//拆分成三个字符串数组for(String s:value){ System.out.print(s);}System.out.println();for(String s:value2){ System.out.print(s);}System.out.println("abcde".contains("abc"));//trueSystem.out.println("abcde".indexOf("a"));//0 参数虽然为整形，但会转为unicode字符System.out.println("abcde".indexOf("bc"));//0 返回首地址System.out.println("abcded".lastIndexOf("d"));//5System.out.println("abcdede".lastIndexOf("de"));//5System.out.println("abcde".isEmpty());//falseSystem.out.println("abcde".length());//5System.out.println("abcde".toUpperCase());//ABCDESystem.out.println("ABCDE".toLowerCase());//abcdeSystem.out.println(" abcde ".trim());//abcdeSystem.out.println("abc".concat("de"));//abcde- 源码
public char charAt(int index) { if ((index < 0) || (index >= value.length)) { throw new StringIndexOutOfBoundsException(index); } return value[index]; } public char[] toCharArray() { // Cannot use Arrays.copyOf because of class initialization order issues char result[] = new char[value.length]; System.arraycopy(value, 0, result, 0, value.length); return result; } // 使用平台默认的编码转换成字节数组，默认gbk，根据虚拟机 public byte[] getBytes(String charsetName) throws UnsupportedEncodingException { if (charsetName == null) throw new NullPointerException(); return StringCoding.encode(charsetName, value, 0, value.length); } public String(byte bytes[]) { this(bytes, 0, bytes.length); } public String(byte bytes[], int offset, int length) { checkBounds(bytes, offset, length); this.value = StringCoding.decode(bytes, offset, length); } public String(byte bytes[], String charsetName) throws UnsupportedEncodingException { this(bytes, 0, bytes.length, charsetName); } public boolean startsWith(String prefix) { return startsWith(prefix, 0); } public boolean startsWith(String prefix, int toffset) { char ta[] = value;//调用该方法的对象的value int to = toffset;//偏移量 char pa[] = prefix.value;//前缀的value int po = 0;//前缀的偏移量 int pc = prefix.value.length;//前缀的总数 // Note: toffset might be near -1>>>1. if ((toffset < 0) || (toffset > value.length - pc)) { return false; } while (--pc >= 0) { if (ta[to++] != pa[po++]) { return false; } } return true; } public boolean endsWith(String suffix) { return startsWith(suffix, value.length - suffix.value.length); } 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; } public String replaceFirst(String regex, String replacement) { return Pattern.compile(regex).matcher(this).replaceFirst(replacement); } public String replaceAll(String regex, String replacement) { return Pattern.compile(regex).matcher(this).replaceAll(replacement); } public String replace(CharSequence target, CharSequence replacement) { return Pattern.compile(target.toString(), Pattern.LITERAL).matcher( this).replaceAll(Matcher.quoteReplacement(replacement.toString())); } 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); } public String substring(int beginIndex, int endIndex) { if (beginIndex < 0) { throw new StringIndexOutOfBoundsException(beginIndex); } if (endIndex > value.length) { throw new StringIndexOutOfBoundsException(endIndex); } int subLen = endIndex - beginIndex; if (subLen < 0) { throw new StringIndexOutOfBoundsException(subLen); } return ((beginIndex == 0) && (endIndex == value.length)) ? this : new String(value, beginIndex, subLen); } public String[] split(String regex, int limit) { /* fastpath if the regex is a (1)one-char String and this character is not one of the RegEx's meta characters ".$|()[{^?*+\\", or (2)two-char String and the first char is the backslash and the second is not the ascii digit or ascii letter. */ char ch = 0; if (((regex.value.length == 1 && ".$|()[{^?*+\\".indexOf(ch = regex.charAt(0)) == -1) || (regex.length() == 2 && regex.charAt(0) == '\\' && (((ch = regex.charAt(1))-'0')|('9'-ch)) < 0 && ((ch-'a')|('z'-ch)) < 0 && ((ch-'A')|('Z'-ch)) < 0)) && (ch < Character.MIN_HIGH_SURROGATE || ch > Character.MAX_LOW_SURROGATE)) { int off = 0; int next = 0; boolean limited = limit > 0; ArrayList<String> list = new ArrayList<>(); while ((next = indexOf(ch, off)) != -1) { if (!limited || list.size() < limit - 1) { list.add(substring(off, next)); off = next + 1; } else { // last one //assert (list.size() == limit - 1); list.add(substring(off, value.length)); off = value.length; break; } } // If no match was found, return this if (off == 0) return new String[]{this}; // Add remaining segment if (!limited || list.size() < limit) list.add(substring(off, value.length)); // Construct result int resultSize = list.size(); if (limit == 0) { while (resultSize > 0 && list.get(resultSize - 1).length() == 0) { resultSize--; } } String[] result = new String[resultSize]; return list.subList(0, resultSize).toArray(result); } return Pattern.compile(regex).split(this, limit); } public String[] split(String regex) { return split(regex, 0); } 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); } } 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); } public int indexOf(int ch) { return indexOf(ch, 0); } public boolean contains(CharSequence s) { return indexOf(s.toString()) > -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); } } public int lastIndexOf(int ch) { return lastIndexOf(ch, value.length - 1); } public boolean isEmpty() { return value.length == 0; } public int length() { return value.length; } 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--; } return ((st > 0) || (len < value.length)) ? substring(st, len) : this; } public String concat(String str) { int otherLen = str.length(); if (otherLen == 0) { return this; } int len = value.length; char buf[] = Arrays.copyOf(value, len + otherLen); str.getChars(buf, len); return new String(buf, true); }阅读全文
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