String、StringBuffer和StringBuilder的区别

从以下几个方面来说：

可变性

String 类中使用 final 关键字修饰字符数组来保存字符串，private final char value[]，所以 String 对象是不可变的。

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    /** The value is used for character storage. */
    private final char value[];
    ...

而 StringBuilder 与 StringBuffer 都继承自 AbstractStringBuilder 类，在 AbstractStringBuilder 中也是使用字符数组保存字符串char[]value 但是没有用 final 关键字修饰，所以这两种对象都是可变的。

补充：在 Java 9 之后，String 、StringBuilder 与 StringBuffer 的实现改用 byte 数组存储字符串 private final byte[] value

StringBuilder 与 StringBuffer 的构造方法都是调用父类构造方法也就是 AbstractStringBuilder 实现的。

 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);
    }
    ...
}

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

    /** use serialVersionUID for interoperability */
    static final long serialVersionUID = 4383685877147921099L;

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

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() {
    }

    /**
     * Creates an AbstractStringBuilder of the specified capacity.
     */
    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }
    ...
}