字符串的学习，有的同学就看看API，记下方法，有的同学看看源代码，还有的同学画画图，自然学的深度是不一样的。
| /** * The {@code String} class represents character strings. All * string literals in Java programs, such as {@code “abc”}, are * implemented as instances of this class. * <p> * Strings are constant; their values cannot be changed after they * are created. String buffers support mutable strings. * Because String objects are immutable they can be shared. For example: * <blockquote><pre> * String str = “abc”; * </pre></blockquote><p> * is equivalent to: * <blockquote><pre> * char data[] = {‘a’, ‘b’, ‘c’}; * String str = new String(data); * </pre></blockquote><p> * Here are some more examples of how strings can be used: * <blockquote><pre> * System.out.println(“abc”); * String cde = “cde”; * System.out.println(“abc” + cde); * String c = “abc”.substring(2,3); * String d = cde.substring(1, 2); * </pre></blockquote> * <p> * The class {@code String} includes methods for examining * individual characters of the sequence, for comparing strings, for * searching strings, for extracting substrings, and for creating a * copy of a string with all characters translated to uppercase or to * lowercase. Case mapping is based on the Unicode Standard version * specified by the {@link java.lang.Character Character} class. * <p> * The Java language provides special support for the string * concatenation operator ( + ), and for conversion of * other objects to strings. String concatenation is implemented * through the {@code StringBuilder}(or {@code StringBuffer}) * class and its {@code append} method. * String conversions are implemented through the method * {@code toString}, defined by {@code Object} and * inherited by all classes in Java. For additional information on * string concatenation and conversion, see Gosling, Joy, and Steele, * <i>The Java Language Specification</i>. |
| String 类代表字符串。Java 程序中的所有字符串字面值（如 “abc” ）都作为此类的实例实现。 字符串是常量；它们的值在创建之后不能更改。字符串缓冲区支持可变的字符串。因为 String 对象是不可变的，所以可以共享。例如： String str = “abc”; 等效于： char data[] = {‘a’, ‘b’, ‘c’}; String str = new String(data); 下面给出了一些如何使用字符串的更多示例： System.out.println(“abc”); String cde = “cde”; System.out.println(“abc” + cde); String c = “abc”.substring(2,3); String d = cde.substring(1, 2); String 类包括的方法可用于检查序列的单个字符、比较字符串、搜索字符串、提取子字符串、创建字符串副本并将所有字符全部转换为大写或小写。大小写映射基于 Character 类指定的 Unicode 标准版。 Java 语言提供对字符串串联符号（”+”）以及将其他对象转换为字符串的特殊支持。字符串串联是通过 StringBuilder（或 StringBuffer）类及其 append 方法实现的。字符串转换是通过 toString 方法实现的，该方法由 Object 类定义，并可被 Java 中的所有类继承。有关字符串串联和转换的更多信息，请参阅 Gosling、Joy 和 Steele 合著的 The Java Language Specification。 |
1.String是个final类
2.String是不可变的字符序列
String对象的字符内容是存储在一个字符数组中的。
private意味着外面无法直接获取字符数组，而且String没有提供value的get和set方法，
final意味着字符数组的引用不可改变，即通过让value指向新的数组对象来实现修改String对象，
而且String也没有提供方法来修改value数组某个元素值，因此字符串的字符数组内容也不可变。
疑问？那么字符串的拼接、字符串的截取、字符串的替换等操作是如何实现的呢？
每次修改都创建一个新的char数组表示修改结果。
3.String对象的创建
String str = “hello”;
String s1 = new String(); // 本质上 this.value = new char[0];
String s2 = new String(String original); //this.value = original.value;
String s3 = new String(char[] a); //this.value = Arrays.copyOf(value, value.length);
String s4 = new String(char[] a,int startIndex,int count)
…….
4.字符串对象是如何存储的
字符串常量存储在字符串常量池，目的是共享
字符串非常量对象存储在堆中。
5.String的拼接
结论：
常量与常量的拼接结果在常量池
只要其中有一个是变量，结果就在堆中
如果拼接的结果调用intern()方法，就在常量池中
6.String对象的比较
==比较的是地址。
equals比较的是字符串的内容，重写了Object的equals方法。
| public boolean equals(Object anObject) { if (this == anObject) { return true; } 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; while (n– != 0) { if (v1[i] != v2[i]) return false; i++; } return true; } } return false; } |
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