为什么一个String值要先赋空值

String teststr=rs.getString(1);
System.out.print("当前编号"+teststr+"\t");
int num=Integer.parseInt(teststr.substring(7))+1;
String nums=""+num;
String testins = null;
if(nums.length()==1){
testins="gasoft-0000"+nums;
String sql="insert into student(stno,name) values(?,?)";
ps=con.prepareStatement(sql);
ps.setString(1, testins);
ps.setString(2, name);
ps.executeUpdate();
}
else if(nums.length()==2){
testins="gasoft-000"+nums;
String sql="insert into student(stno,name) values(?,?)";
ps=con.prepareStatement(sql);
ps.setString(1, testins);
ps.setString(2, name);
ps.executeUpdate();
}
else if(nums.length()==3){
testins="gasoft-00"+nums;
String sql="insert into student(stno,name) values(?,?)";
ps=con.prepareStatement(sql);
ps.setString(1, testins);
ps.setString(2, name);
ps.executeUpdate();
}
else if(nums.length()==4){
testins="gasoft-0"+nums;
String sql="insert into student(stno,name) values(?,?)";
ps=con.prepareStatement(sql);
ps.setString(1, testins);
ps.setString(2, name);
ps.executeUpdate();
}
else if(nums.length()==5){
testins="gasoft-"+nums;
String sql="insert into student(stno,name) values(?,?)";
ps=con.prepareStatement(sql);
ps.setString(1, testins);
ps.setString(2, name);
ps.executeUpdate();
}
else{
System.out.println("超出取值范围请坐调整");
String sql="insert into student(stno,name) values(?,?)";
ps=con.prepareStatement(sql);
ps.setString(1, "error");
ps.setString(2, name);
ps.executeUpdate();
}
System.out.println("编号可赋值"+testins);
rs.close(); //关闭ResultSet对象
ps.close(); //关闭Statement对象
con.close();
}

“String testins = null;”不赋值不可以吗 ？

相关了解……