如何访问对象列表中的元素

Question

I'm defining an ArrayList as such

ArrayList <Object> p = new ArrayList<Object>();

now to the ArrayList I'm adding like so

String source = "hi";
int x,y,year = 0;
p.add(new Object(source,x,y,year));
p.add(new Object(source+"hello",x+1,y+1,year+1));

Now in the main method, I want to access a specific element within the the list of Objects. How would I do that?

When I print it out in the main method, it outputs

[source,x,y,year, source1,x1,y1,year1] 

However, I want just the source from the first element. So output just "hi" as type String not as a object.

Answer 1

One way to achieve your desired result is by putting the arguments in a list by using Arrays.asList() . Something like this.

ArrayList<List> listOfList = new ArrayList<List>();
        
String greetings = "hi";
int x = 0, y = 0, year = 0;
        
listOfList.add(Arrays.asList(greetings, x, y, year));
listOfList.add(Arrays.asList(greetings + " hello", ++x, ++y, ++year));
        
//Get and print list of the first list.
List listA = listOfList.get(0);
for(Object obj : listA) {
    System.out.print(obj + " ");
}
System.out.println();
//Prints: "hi 0 0 0 "
        
//Print the second list.
System.out.println(listOfList.get(1));
//Prints: "[hi hello, 1, 1, 1]"

Answer 2

p.get(0) would get you the first object in the list.

Java is object oriented. Objects define what you can do with them. They don't "have" a property unless whomever wrote the class decides that it should have it.

Thus, your question is unanswerable without the source code of whatever Object is in your question. Presumably it has a .getSource() method which returns it. If it does: p.get(0).getSource() would do the job. If it doesn't, find the method that does that job, and if it doesn't exist, write it, and if you didn't write that class, then you can't do it.