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How to access a field in a (specific) list element

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• Review: How to access a field in a specific list element in Java
  • To make the material more concrete to explain, I will use the following (simple) List object:

    // File: /home/cs255001/demo/asm/4-linked-list/List.java public class List { int value; // int typed variable takes up 4 bytes List next; // reference variable contains an address, also 4 bytes // instance methods omitted - not relevant for discussion }

    I will assume that a linked list has been created and the head element of the list is stored in this variable:

    List head;

  • Recall that this is the loop you used in CS171 to access all the List elements and print the value in each list element:

    List ptr; ptr = head; while ( ptr != null ) { System.out.println( ptr.value ); ptr = ptr.next; }

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/asm/4-linked-list/ListMain.java

    How to run the program:

    To compile:   javac ListMain.java To run:          java ListMain

    Output:

    cheung@aruba> java ListMain 11 22 33 44 55

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  • Observe how a high level language program process a linked list:

    The program uses a List typed variable ptr to reference (= point) to the current List object: The initialization ptr = head will make ptr point to the first list element: The assignment ptr = ptr.next will make ptr point to the list element that follows the currently pointed element: Or: (after cleaning up the above figure):

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  • For the Java program and the explanation on what the Java statements do, you can see that:

    Some current list element is pointed to by a reference variable (in our program, the variable is ptr) The program statements uses this reference variable (ptr) to access the fields in that specific list element: ptr.value accesses the field "value" in the list element pointed to by ptr ptr.next accesses the field "next" in the list element pointed to by ptr

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• How to access a field in a (specific) list element
  • Information given to you to access a specific linked list element (= object):

    You are given the address of the linked list object

    Example: we are given the address 7000 - which is the start address of some list object

    Notice that:

    The start address of a list object is similar to the base address of an array I.e.: the fields in a list object always has a fixed offset from the base address of a list object !!!

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  • To access the fields in a list object, we can use the base address + offset approach (that we used to access array elements):

    The offsets used in linked list will be "computed" differently

    In fact:

    The offsets used to access fields in a list element will depend on the structure of the List object !!!

    Note:

    The alignment requirements imposed on int and short typed fields in a list object will leave gaps between data fields in a list structure Gaps are memory bytes that are not used... Gaps will make the offset "computation" very difficult Therefore: I will therefore use internal structures that can packed tightly (= no gaps) to simplify the discussion

    I will now show you how to "compute" the offsets to access various data fields defined in a linked list structure using 2 different List classes

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• Accessing fields in a List object - example 1
  • Consider the List object in our example above which is defined as:

    // File: /home/cs255001/demo/asm/4-linked-list/List.java public class List { int value; // int typed variable takes up 4 bytes List next; // reference variable contains an address, also 4 bytes // instance methods omitted - not relevant for discussion }

    This picture shows the placement of the data fields value and next in a list element:

    Therefore:

    To access the value field in a list object, we use the offset 0 (zero) from the base address of the list element: address(value field) = base address(list element) + 0 To access the next field in a list object, we use the offset 4 from the base address of the list element: address(next field) = base address(list element) + 4

    I will show you an assembler program example later....

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• Accessing fields in a List object - example 2
  • Consider the List object that is defined as:

    public class List { int value1; // int typed variable takes up 4 bytes short value2; // short typed variable takes up 2 bytes short value3; // short typed variable takes up 2 bytes List next; // reference variable contains an address, also 4 bytes // instance methods omitted - not relevant for discussion }

    This picture shows the placement of the data fields value and next in a list element:

    Therefore:

    To access the value1 field in a list object, we use the offset 0 (zero) from the base address of the list element: address(value1 field) = base address(list element) + 0 To access the value2 field in a list object, we use the offset 4 from the base address of the list element: address(value2 field) = base address(list element) + 4 To access the value3 field in a list object, we use the offset 6 (4+2) from the base address of the list element: address(value3 field) = base address(list element) + 6 To access the next field in a list object, we use the offset 8 (4 + 2 + 2) from the base address of the list element: address(next field) = base address(list element) + 8

    I will show you an assembler program example later....