为什么 String 的内容不可不变 StringBuffer 内容可变

研读了java代码 String.java 文件中实现

使用了 final 关键字 内容不可 over write

  private final char value[];

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;

/**
* Class String is special cased within the Serialization Stream Protocol.
*
* A String instance is written into an ObjectOutputStream according to
* <a href="{@docRoot}/../platform/serialization/spec/output.html">
* Object Serialization Specification, Section 6.2, "Stream Elements"</a>
*/
private static final ObjectStreamField[] serialPersistentFields =
new ObjectStreamField[0];

/**
* 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;
}

 StringBuffer 中 使用了transient 关键字 所以变量不会被序列化

public final class StringBuffer
extends AbstractStringBuilder
implements java.io.Serializable, CharSequence
{

/**
* A cache of the last value returned by toString. Cleared
* whenever the StringBuffer is modified.
*/
private transient char[] toStringCache;

/** use serialVersionUID from JDK 1.0.2 for interoperability */
static final long serialVersionUID = 3388685877147921107L;

/**
* Constructs a string buffer with no characters in it and an
* initial capacity of 16 characters.
*/
public StringBuffer() {
super(16);
}