A detailed description of the String class in Java

Examples of classes of embodiment 1.String

1. Direct assignment

	String str = "hello world";  //str是一个对象的引用，所以“hello world”应该保存在堆内存中
	System.out.println(str);

2. Conventional method (constructor)

	//因为String是一个类，所以存在构造方法
	String str = new String("hello world");   //通过构造方法赋值
	System.out.println(str);

2. String string comparison

1. If there are two int type variables, by determining whether the same ==determination

	int x = 10;
	int y = 10;
	System.out.println(x==y);   //true

2. If two String comparison, "==" comparison, the comparison is a reference to the type of address

	String str = "hello world";
	String str1 = "hello world";
	String str2 = new String("hello world");

	System.out.println(str == str1);  //true
	System.out.println(str2 == str1); //false

3. The method of comparing strings with content equals

	String str1 = "hello world";
	String str2 = new String("hello world");
	System.out.println(str1.equals(str2));   //true

to sum up:

• Value "==" operator is used to compare two variables are equal, the basic types, that is, comparison of the numerical size ; a reference to a data type, the comparison is saved in a Java String class described in detail and will not compare the contents
• When comparing the contents of the string, the String class provides equals () method, a case-sensitive; use ** equalsIngoreCase () ** case insensitive
• When the direct assignment, the string is determined whether there is an object in the pool, if any, to the direct reference to an object pool
• Pooling the equivalent of a one-dimensional array
• If you are using the constructor, then open up a new space

3. String anonymous objects (not from the name of the object)

All string constants" " anonymous objects, are the String class

	String str1= "hello";
	String str = new String("hello");
	System.out.println(str1.equals(str));
	System.out.println("hello".equals(str));

• "String str =" hello ";" is an anonymous String class object is provided with a name on nature, but also to preserve the heap memory
• Any string constants are anonymous String object, the object will never be null

4. Examples of the difference

1. Direct assignment

	String str = "hello";
	String str2 = "hello";
	Stirng str1 = "hello";
	System.out.println(str == str1);   //true
	System.out.println(str == str2);   //true
	System.out.println(str1 == str2);  //true
	//str, str1 , str2三个地址指向同一块空间

In fact, the underlying JVM will automatically maintain a string constant pool (array of objects) . If a direct assignment to instantiate an object, first find out whether there is a specified content object in the object pool, and if so, direct references; on the contrary, the newly created; in fact, is the use of design patterns in the Flyweight

2. The method of construction using

	String str = new String("hello");
	String str1 = "hello";
	System.out.println(str == str1);  //false 

• Using construction methods will open up two heap space
• First put the anonymous object "hello", is a string constant to open up, and then a new real object; points out that the new true object, that memory will become an anonymous object space junk
• Instantiated object and not saved in the memory pool

	//入池操作
	//把String对象入池（如果池中有，引用指向它；如果没有，入池加指向）
	String str = "hello";
	String str1 = new String("hello").intern();
	System.out.println(str == str1);   //true

The string can not be changed

• Once defined string, immutable
• All language is a fixed length of the string to achieve the greatest flaw, all character array, the array. When defining a string constant, its contents immutable

	String str = "hello";
	str = str +" world";
	str += "!!!";
	System.out.println(str);   //hello world!!!
	//4块垃圾空间
	//堆中常量的值无法修改，但是栈的指向一直发生变化

Change the above string, the string is subject to change, instead of a string constants
illustrated as follows: