String str="";
for(int i=0;i<rtstr1.length;i++){
  str=str+rtstr1[i]+";"
}

------解决方案--------------------
Java code

for (int ln = 0; ln < list1.size(); ln++) { 
Object[] obj = (Object[]) list1.get(ln); 
for (int w = 0; w < bj.length; w++) { 
String[] rtstr1 = new String[w]; 
if (obj[w] != null) { 
rtstr1[w] = (String) obj[w]; 
String str="";
for(int i=0;i<rtstr1.length;i++){
  str+=rtstr1[i]+";"
}
System.out.println(str);
} else { 
rtstr1[w] = "NULL"; 
} 
    } 
}

------解决方案--------------------
Java code

for (int ln = 0; ln < list1.size(); ln++) { 
Object[] obj = (Object[]) list1.get(ln); 
for (int w = 0; w < bj.length; w++) { 
String[] rtstr1 = new String[w]; 
if (obj[w] != null) { 
rtstr1[w] = (String) obj[w]; 
String str="";
for(int i=0;i<rtstr1.length;i++){
  str+=rtstr1[i]+";"
}
System.out.println(str);
} else { 
rtstr1[w] = "NULL"; 
} 
    } 
}

------解决方案--------------------
String str=null;
for(int i=0;i<list1.size();++i)
{
  str=str+rtstr1[i]+";";
}

------解决方案--------------------
Java code

String str=null;
for(int i=0;i <list1.size();i++) 
{ 
  str=str+rtstr1[i]+";"; 
}

------解决方案--------------------
Java code

public static void main(String[] args) {

        List<Object[]> list = new ArrayList<Object[]>();
        list.add(new Object[] { "a", "b", "c", null });
        list.add(new Object[] { null, "b", "c", null });

        int objCount = 0;
        for (Object[] obj : list) {
            String str = "";
            int oCount = 0;
            for (Object o : obj) {
                str += "obj[" + oCount++ + "]=" + (o != null ? ((String) o + "   ") : ("NULL" + "   "));
            }
            System.out.println("Object[" + objCount++ + "]:" + '\t' + str);
        }
    }

------解决方案--------------------
Java code

for (int ln = 0; ln < list1.size(); ln++) { 
  Object[] bj = (Object[]) list1.get(ln); 
  for (int w = 0; w < bj.length; w++) { 
    String[] rtstr1 = new String[w]; //这里我不明白LZ想做什么,这样写每次循环都会声明一个长度为w的字符串数组.而且作用域只在本循环中.无意义
    if (obj[w] != null) { 
      rtstr1[w] = (String) obj[w]; 
    } else { 
      rtstr1[w] = "NULL"; 
    } 
  } 
} 


String[] rtstr1 = new String[list1.size()];
for (int ln = 0; ln < list1.size(); ln++) { 
  Object[] bj = (Object[]) list1.get(ln); 
  StringBuilder rtstr14Bj = new StringBuilder();//用于存放以";"分割的bj中的元素.
  for (int w = 0; w < bj.length; w++) { 
    String[] rtstr1 = new String[w]; 
     if (bj[w] != null) { 
      rtstr14Bj.append(bj[w]);
      rtstr14Bj.append(";"); 
     } else { 
      rtstr14Bj.append("null");
      rtstr14Bj.append(";"); 
     } 
  } 
  rtstr1[ln] = rtstr14Bj.toString().substring(0,rtstr14Bj.length());
}