String:
源码：
public final class String
implements java.io.Serializable, Comparable, CharSequence {
/** The value is used for character storage. */
private final char value[];
/**
* Initializes a newly created {@code String} object so that it represents
* an empty character sequence. Note that use of this constructor is
* unnecessary since Strings are immutable.
*/
public String() {
this.value = "".value;
}
/**
* Initializes a newly created {@code String} object so that it represents
* the same sequence of characters as the argument; in other words, the
* newly created string is a copy of the argument string. Unless an
* explicit copy of {@code original} is needed, use of this constructor is
* unnecessary since Strings are immutable.
*
* @param original
* A {@code String}
*/
public String(String original) {
this.value = original.value;
this.hash = original.hash;
}
/**
* Allocates a new {@code String} so that it represents the sequence of
* characters currently contained in the character array argument. The
* contents of the character array are copied; subsequent modification of
* the character array does not affect the newly created string.
*
* @param value
* The initial value of the string
*/
public String(char value[]) {
this.value = Arrays.copyOf(value, value.length);
}
}
总结：
1. 类final修饰的，可见String 是不可继承的
2. value也是final修饰的，可见，一经创建，不可修改
3. 新建对象有几种方法new String(), new String(字符串对象),new String(字符数组）等。
StringBuffer:
源码：
public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
/**
* Constructs a string buffer with no characters in it and an
* initial capacity of 16 characters.
*/
public StringBuffer() {
super(16);
}
/**
* Constructs a string buffer with no characters in it and
* the specified initial capacity.
*
* @param capacity the initial capacity.
* @exception NegativeArraySizeException if the {@code capacity}
* argument is less than {@code 0}.
*/
public StringBuffer(int capacity) {
super(capacity);
}
/**
* Constructs a string buffer initialized to the contents of the
* specified string. The initial capacity of the string buffer is
* {@code 16} plus the length of the string argument.
*
* @param str the initial contents of the buffer.
*/
public StringBuffer(String str) {
super(str.length() + 16);
append(str);
}
@Override
public synchronized StringBuffer append(Object obj) {
toStringCache = null;
super.append(String.valueOf(obj));
return this;
}
@Override
public synchronized StringBuffer append(String str) {
toStringCache = null;
super.append(str);
return this;
}
@Override
public synchronized String toString() {
if (toStringCache == null) {
toStringCache = Arrays.copyOfRange(value, 0, count);
}
return new String(toStringCache, true);
}
}
总结：
可以看到StringBuffer是线程安全的，当修改StringBuffer对象调用append方法是加锁的
如果想转换成String 对象，可以调用 toString() 方法
StringBuffer 是可变的，可以对StringBuffer内容进行更改
StringBuilder：
源码：
public final class StringBuilder
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{
public StringBuilder(int capacity) {
super(capacity);
}
/**
* Constructs a string builder initialized to the contents of the
* specified string. The initial capacity of the string builder is
* {@code 16} plus the length of the string argument.
*
* @param str the initial contents of the buffer.
*/
public StringBuilder(String str) {
super(str.length() + 16);
append(str);
}
/**
* Constructs a string builder that contains the same characters
* as the specified {@code CharSequence}. The initial capacity of
* the string builder is {@code 16} plus the length of the
* {@code CharSequence} argument.
*
* @param seq the sequence to copy.
*/
public StringBuilder(CharSequence seq) {
this(seq.length() + 16);
append(seq);
}
/**
* Appends the string representation of the {@code Object} argument.
*
* The overall effect is exactly as if the argument were converted
* to a string by the method {@link String#valueOf(Object)},
* and the characters of that string were then
* {@link #append(String) appended} to this character sequence.
*
* @param obj an {@code Object}.
* @return a reference to this object.
*/
public AbstractStringBuilder append(Object obj) {
return append(String.valueOf(obj));
}
/**
* Appends the specified string to this character sequence.
*
* The characters of the {@code String} argument are appended, in
* order, increasing the length of this sequence by the length of the
* argument. If {@code str} is {@code null}, then the four
* characters {@code "null"} are appended.
*
* Let n be the length of this character sequence just prior to
* execution of the {@code append} method. Then the character at
* index k in the new character sequence is equal to the character
* at index k in the old character sequence, if k is less
* than n; otherwise, it is equal to the character at index
* k-n in the argument {@code str}.
*
* @param str a string.
* @return a reference to this object.
*/
public AbstractStringBuilder append(String str) {
if (str == null)
return appendNull();
int len = str.length();
ensureCapacityInternal(count + len);
str.getChars(0, len, value, count);
count += len;
return this;
}
@Override
public String toString() {
// Create a copy, don't share the array
return new String(value, 0, count);
}
}
抽象类AbstractStringBuilder ：
abstract class AbstractStringBuilder implements Appendable, CharSequence {
/**
* The value is used for character storage.
*/
char[] value;
/**
* The count is the number of characters used.
*/
int count;
/**
* This no-arg constructor is necessary for serialization of subclasses.
*/
AbstractStringBuilder() {
}
@Override
public abstract String toString();
}
总结：
1.可以看到StringBuilder 和StringBuffer方法很相似，区别是StingBuilder不是线程安全的
2.相同点 都继承抽象类 AbstractStringBuilder， 都是可变的
最后总结：
都有toString()方法 在接口CharSequence中定义了toString()方法
针对以上String,StringBuffer,与StringBuilder 稍做总结：
String是不可变的，StringBuffer，StringBuilder是可变的
StringBuffer,StringBuilder 都有 toString()方法可以转化未String对象
StringBuffer与StringBuilder都继承同一抽象类AbstractStringBuilder
StringBuffer是线程安全的，StingBuilder是非线程安全的
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