JAVA類String
- 2019 年 12 月 15 日
- 筆記
今天要講的是JDK中的String類了，相信大家對這個類特別的熟悉，那今天話不多說，直接講一些常用的方法。
首先有個內部類：
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) { // 某些字母大寫是無法比較的，所以這裡還是得判斷一下 // No overflow because of numeric promotion return c1 - c2; } } } } return n1 - n2; } /** Replaces the de-serialized object. */ private Object readResolve() { return CASE_INSENSITIVE_ORDER; } } //再看下這個內部類在String類中的使用 public static final Comparator<String> CASE_INSENSITIVE_ORDER = new CaseInsensitiveComparator(); public int compareToIgnoreCase(String str) { return CASE_INSENSITIVE_ORDER.compare(this, str); } String str1 = "abba"; String str2 = "Abba"; str1.compareToIgnoreCase(str2) --- 0 完全匹配 String str1 = "abba"; String str2 = "Abba"; str1.compareToIgnoreCase(str2) --- 1 包含 String str1 = "abba"; String str2 = "Abbac"; str1.compareToIgnoreCase(str2) --- -1 不匹配
屬性：
private final char value[]; // 很明白了 用value存 private int hash; // constructor public String(String original) { this.value = original.value; this.hash = original.hash; } // // 順便看下這個hash public int hashCode() { int h = hash; if (h == 0 && value.length > 0) { char val[] = value; for (int i = 0; i < value.length; i++) { h = 31 * h + val[i]; // val[0]*31^(n-1) + val[1]*31^(n-2) + ... + val[n-1] } hash = h; } return h; } // val[0]*31^(n-1) + val[1]*31^(n-2) + ... + val[n-1] // n是字元串的長度 很明顯n比較大的時候就會溢出 這樣也證明hashCode相同的String串，其內容不一定相同
METHOD
// 方法 public int length() { return value.length; // 返回char數組的length 很簡單 包裝成方法了 } public boolean isEmpty() { return value.length == 0; } // 返回指定下標的字元 public char charAt(int index) { if ((index < 0) || (index >= value.length)) { throw new StringIndexOutOfBoundsException(index); } return value[index]; // 還是數組操作 } // 把字元串放入char數字 void getChars(char dst[], int dstBegin) { System.arraycopy(value, 0, dst, dstBegin, value.length); } // 把字元串中srcBegin到srcEnd - 1放到從char數組dstBegin開始往後的空間 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); } // 比較這個是重寫的Object方法 public boolean equals(Object anObject) { // 地址一樣返回true if (this == anObject) { return true; } // 地址不一樣如果是String那麼會繼續比較內容 if (anObject instanceof String) { String anotherString = (String)anObject; int n = value.length; if (n == anotherString.value.length) { char v1[] = value; char v2[] = anotherString.value; int i = 0; //循環比較 while (n-- != 0) { if (v1[i] != v2[i]) return false; i++; } return true; } } return false; } // 以什麼開始 從字元串的toffset開始比較 public boolean startsWith(String prefix, int toffset) { char ta[] = value; int to = toffset;// 從to開始 char pa[] = prefix.value; int po = 0; int pc = prefix.value.length; // Note: toffset might be near -1>>>1. if ((toffset < 0) || (toffset > value.length - pc)) { return false; } // 一個一個char字元匹配 while (--pc >= 0) { if (ta[to++] != pa[po++]) { return false; } } return true; } // 返回ch字元（ACII碼 ）第一次出現的下標 不匹配返回-1 public int indexOf(int ch) { return indexOf(ch, 0); } // 返回字元串在當前字元串中出現的位置 不匹配返回-1 public int indexOf(String str) { return indexOf(str, 0); } 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); } static int indexOf(char[] source, int sourceOffset, int sourceCount, char[] target, int targetOffset, int targetCount, int fromIndex) { if (fromIndex >= sourceCount) { return (targetCount == 0 ? sourceCount : -1); } if (fromIndex < 0) { fromIndex = 0; } if (targetCount == 0) { return fromIndex; } char first = target[targetOffset]; int max = sourceOffset + (sourceCount - targetCount); // fromIndex從第幾個開始 for (int i = sourceOffset + fromIndex; i <= max; i++) { /* Look for first character. */ if (source[i] != first) { // 先找到第一個匹配的字元 while (++i <= max && source[i] != first); } /* Found first character, now look at the rest of v2 */ if (i <= max) { int j = i + 1; int end = j + targetCount - 1; // 開始比較 for (int k = targetOffset + 1; j < end && source[j] == target[k]; j++, k++); //最後一個也相等才會到這 if (j == end) { // 返回下標 return i - sourceOffset; } } } return -1; } // 以什麼結尾其實就是用的startsWith取最後 public boolean endsWith(String suffix) { return startsWith(suffix, value.length - suffix.value.length); } // 這個方法是從字元常量池裡拿String public native String intern(); String str1 = new String("abba"); String str2 = "abba"; System.out.println(str1 == str2); // false String str3 = str1.intern(); System.out.println(str2 == str3); // true // trim() 去除字元串兩邊空格 String str = " ssad"; str.trim() = "ssad" // substring 切分字元串從beginIndex開始到endIndex - 1 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); } // join() 拼接 List<String> list = new ArrayList(); list.add("aa"); list.add("aa"); list.add("aa"); list.add("aa"); list.add("aa"); String.join(",",list); System.out.println(list); // [aa, aa, aa, aa, aa] // substring 切分字元串從beginIndex開始到endIndex - 1 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); } // split() 切分 String str1 = "qw-sdad-dsa"; String[] bs = str1.split("-"); // bs[0] = qw bs[0] = wdad bs[2] = dsa //replace() 替換 String str1 = "qw-sdad-dsa"; System.out.println(str1.replace("d", "D")); // qw-sDaD-Dsa
