java.lang.Object
  |
  +-- java.lang.StringBuilder

public final class StringBuilder extends Object implements Serializable, CharSequence

String str = "abc" + "def" + "ghi";

append
public StringBuilder append(String str)

StringBuilder sb = new StringBuilder();
sb.append("abc");
sb.append("def");
sb.append("ghi");

Strint str = new String(sb);  // "abcdefghi"

StringBuilder append(boolean b)
StringBuilder append(char c)
StringBuilder append(char[] str)
StringBuilder append(char[] str, int offset, int len)
StringBuilder append(CharSequence s)
StringBuilder append(CharSequence s, int start, int end)
StringBuilder append(double d)
StringBuilder append(float f)
StringBuilder append(int i)
StringBuilder append(long lng)
StringBuilder append(Object obj)
StringBuilder append(StringBuffer sb)

サンプルプログラム

class testStringBuilder1{
  public static void main(String args[]){
    dispHantei("山田", 89);
    dispHantei("鈴木", 45);
    dispHantei("伊藤", 67);
    dispHantei("加藤", 92);
  }

  private static void dispHantei(String name, int tokuten){
    StringBuilder sb = new StringBuilder();

    sb.append(name);
    sb.append("さんの成績は");
    sb.append(tokuten);
    sb.append("点です。結果は");
    if (tokuten > 75){
      sb.append("合格");
    }else{
      sb.append("不合格");
    }
    sb.append("です。");

    System.out.println(new String(sb));
  }
}

[xxxxxxxx@dddddddddd StringBuilder]$ javac testStringBuilder1.java
[xxxxxxxx@dddddddddd StringBuilder]$ java testStringBuilder1
山田さんの成績は89点です。結果は合格です。
鈴木さんの成績は45点です。結果は不合格です。
伊藤さんの成績は67点です。結果は不合格です。
加藤さんの成績は92点です。結果は合格です。
[xxxxxxxx@dddddddddd StringBuilder]$ 

insert
public StringBuilder insert(int offset, String str)

StringBuilder insert(int offset, boolean b)
StringBuilder insert(int offset, char c)
StringBuilder insert(int offset, char[] str)
StringBuilder insert(int index, char[] str, int offset, int len)
StringBuilder insert(int dstOffset, CharSequence s)
StringBuilder insert(int dstOffset, CharSequence s, int start, int end)
StringBuilder insert(int offset, double d)
StringBuilder insert(int offset, float f)
StringBuilder insert(int offset, int i)
StringBuilder insert(int offset, long l)
StringBuilder insert(int offset, Object obj)

サンプルプログラム

class testStringBuilder2{
  public static void main(String args[]){
    StringBuilder sb = new StringBuilder();
    sb.append("こんにちは。元気です。");
    sb.insert(5, "鈴木さん");

    System.out.println(new String(sb));
  }
}

[xxxxxxxx@dddddddddd StringBuilder]$ javac testStringBuilder2.java
[xxxxxxxx@dddddddddd StringBuilder]$ java testStringBuilder2
こんにちは鈴木さん。元気です。
[xxxxxxxx@dddddddddd StringBuilder]$