Java's Memory Management

Transcription

1 Java's Memory Management Oliver W. Layton CS231: Data Structures and Algorithms Lecture 04, Fall 2018 Wednesday September 12

2 Java %p of the day: Common compila%on errors Class name must MATCH the filename of the.java file When running your java code, you cannot have an extension like.java or.class. java MyClass is ok, but the following fails: java MyClass.java or java MyClass.class. Recompile (build your code) with javac aher making changes to your code to see them reflected when you run your program. You should build ALL your source code files in your project for your program to run: e.g. javac Class1.java Class2.java Class3.java You always need a main method in the class that you call via java MyClass. You need to make sure you write your source code with a text editor or Java IDE, NOT MicrosoH Word.

3 Alloca&ng memory: primi&ves and objects Primi&ves: int i; // nothing allocated in memory for i i = 3 // compiler allocates 4 bytes space in memory for a single int Objects and new keyword: Die d; // nothing allocated in memory for d d = new Die(); // compiler allocates memory for the object This includes the object's internal fields, which are given default values.

4 Alloca&ng memory: default values Let's say we had a String field in an object that we just instan9ated. Can anyone guess the default value? null (same for other objects), which is analogous to None in Python. Primi9ves are usually ini9alized to 0.

5 Java uses two data structures to store items in memory: stack and heap

6 1. When Java uses a stack Primi&ve types inside some method Primi&ve type parameters inside some method Names of methods References to objects (not objects themselves). Let's draw the stack out on the board for a concrete example method: /*initialfaceval is 5*/ public void maketwodice(int initialfaceval) { Die d1 = new Die(); Die d2 = new Die(initialFaceVal); int topface = d1.getvalue(); } Die d3 = d2;

7 2. When Java uses a heap Objects Fields of object Let's draw this out on board /*initialfaceval is 5*/ public void maketwodice(int initialfaceval) { Die d1 = new Die(); Die d2 = new Die(initialFaceVal); int topface = d1.getvalue(); } Die d3 = d2;

8 Local variables vs. instance variables Local variables: temporary variables that exist only when a method is executed. num1, num2, and num3 are all local variables: public void myfun(int num1) { int num2 = 2; Integer num3 = new Integer(5); } Instance variables: data fields of an object. Exist in memory as long as owning object is referenced in memory. How many objects are there on the board? References? What if we add line d1 = d2;?

9 Garbage collec+on and automa+c memory management

10 Life%me A variable's life%me: when the variable can be referenced/accessed within a Java program. Local variables: /me between when a method is called and when it returns. Objects: /me between space being allocated via the new keyword and when it gets garbage collected.

11 Scope A variable's scope: where the variable can be referenced/accessed within a program. Local variables: the method within which the variable is defined, from the point of declara9on to the point where it can no longer be referenced (e.g. a=er method returns). Objects: depends on the public, private and protected modifiers. What do these mean again? NOTE: Loop control variables have their own local scope. for (int i = 0; i < 5; i++) { System.out.println(i); } System.out.println(i); // fails to compile

12 Memory Summary Local variables Instance variables Loca%on stack heap Life%me Scope during method execu%on from declara%on to end of method from instan%a%on un%l completely dereferenced (garbage collec%on) depends on access modifier: public, protected, private

13 Deep vs shallow copies /*initialfaceval is 5*/ public void maketwodice(int initialfaceval) { Die d1 = new Die(); Die d2 = new Die(initialFaceVal); int topface = d1.getvalue(); } Die d3 = d2; Die d3 = d2; doesn't actually copy any of d2's data into d3. Assignment operator (=) makes a shallow copy: make a REFERENCE (to d2), don't copy the object data.

14 Shallow copy confusion We change d2 -> we change d3: //... /*facevalue is 5 in d2*/ Die d3 = d2; d2.setvalue(2); System.out.println(d2.toString()); // Face value: 2 System.out.println(d3.toString()); // Face value: 2 System.out.println(d2 == d3); // true... why?

15 Deep copies (cloning) Deep copy (clone): A real copy of an object. All data fields from one object are copied to another. They are independent and do not reference one another. Let's draw a picture.

16 Steps to create a Die deep copy (clone) 1. Create a method with the signature public Die clone(). 2. In the method body, create a new Die object that will be the deep copy (using new keyword). 3. Use set methods to copy over the field values one-by-one from the current instance to the deep clone object. 4. Return the deep copy.

17 Let's add a clone() method to our Die class to prac1ce making a deep copy.