A question about the object type conversion

I now need is : to define a type A, can be assigned to almost any type A, I can pass as a result of the A to another place, and then type in other places and then convert it back - using casts - use it.
I put this A set Object, String pass when there is no problem , place the receiver using casts (String). But ran into a HashMap , compile time warning , though does not affect operation. I would like to use a program without warning .

警告: [unchecked] 未经检查的转换
              Map<String, String> urls = (HashMap<String, String>) chunk.body;
    需要: HashMap<String,String>
    找到:    Object

Can generics do not know , T, V , etc. What does it mean , what's the difference ?
------ Solution ---------------------------------------- ----
this warning can not be avoided , the parent wants to turn into any subclass will have this warning , as long as you get to know your way
what type will ensure the program is running will not throw ClassCastException on the line.
------ Solution ---------------------------------------- ----
after receiving how to use it ? Object.? I want to pass HashMap !                                  
          
directly set to take the time to go get on the line ah you tried it?                            
        

  import java.util.HashMap;
  
  public class NewMainTest<T> {
  
      private T obj;
  
      public void setObj(T obj) {
          this.obj = obj;
      }
  
      public T getObj() {
          return obj;
      }
  
      /**
       * @param jsonArray the command line arguments
       */
      public static void main(String args[]) throws Exception {
          NewMainTest nmt = new NewMainTest<>();
          HashMap<String, String> map = new HashMap<>();
          nmt.setObj(map);
          HashMap<String, String> map1 = (HashMap<String, String>) nmt.getObj();
          //map1.get;
      }
  }
  

warning ah.                      
      
it will only add a comment            how to add a comment ?          
@ SuppressWarnings ("unchecked") can be added to the front line of code , only the line of work , ah, easy to use !  


ah see SuppressWarnings target range @ Target (value = {TYPE, FIELD, METHOD, PARAMETER, CONSTRUCTOR, LOCAL_VARIABLE})
anything can be added in addition to the front of the package and annotation
------ For reference only ----------------------- ----------------

  public class A<T>
  {
  	private T obj;
  
  	public void setObj(T obj) {
  		this.obj = obj;
  	}
  
  	public T getObj() {
  		return obj;
  	}
  
  	
  }
  

requires you to specify certain generic type , if you want to be able to pass any object A is necessary to specify the generic Object, for example : A