String:

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;
    ...   
}

StringBuffer:

 public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
    @Override
    public synchronized StringBuffer append(Object obj) {
        toStringCache = null;
        super.append(String.valueOf(obj));
        return this;
    }
}

StringBuilder:

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
　@Override
    public StringBuilder append(Object obj) {
      return append(String.valueOf(obj));
   }

}

java 基礎類型