1 / 19

Today

Today. Quiz 1 solutions posted on the Grading page. Assignment 1 solution posted Wednesday after 7pm. Assignment 2 is posted – due Oct. 24, 7pm. Continue with Pointers in Java: Aliasing pointers. Passing by Reference. Null pointers. Aliasing Objects - Array Example. 5.

jade-estes
Télécharger la présentation

Today

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Today • Quiz 1 solutions posted on the Grading page. • Assignment 1 solution posted Wednesday after 7pm. • Assignment 2 is posted – due Oct. 24, 7pm. • Continue with Pointers in Java: • Aliasing pointers. • Passing by Reference. • Null pointers. CISC124 - Prof. McLeod

  2. Aliasing Objects - Array Example 5 int[] first = {1, 2, 3, 4, 5}; int[] second = {10, 20, 30, 40, 50, 60, 70}; 1 2 3 4 5 10 20 30 40 50 60 70 0480ff 0960ff int[] first 0480ff int[] second 0960ff .length .length 7 CISC124 - Prof. McLeod

  3. Aliasing Objects - Array Example, Cont. 5 second = first; // Aliasing! 1 2 3 4 5 10 20 30 40 50 60 70 0480ff 0960ff int[] first 0480ff int[] second 0480ff .length .length 7 CISC124 - Prof. McLeod

  4. Aliasing Objects - Array Example, Cont. 5 // after garbage collection Poof! 1 2 3 4 5 0480ff int[] first 0480ff int[] second 0480ff .length CISC124 - Prof. McLeod

  5. Aside – “Garbage Collection” in Java • Some computer programming languages require you to indicate when you are done with variables so the memory they are occupying can be released back to the OS. Called “Garbage Collection”. • (Fortunately!) Java has an automatic Garbage Collection system: • Variables are garbage collected once you move outside their scope. • Object contents are garbage collected when there are no pointers pointing to the contents. CISC124 - Prof. McLeod

  6. Aliasing Objects - Array Example, Cont. 5 first[4] = 500; // second[4] is also 500 1 2 3 4 500 0480ff int[] first 0480ff int[] second 0480ff .length CISC124 - Prof. McLeod

  7. Aliasing Objects - Array Example, Cont. • So, setting one array to equal another as in: array1 = array2; sets array1 to point to the same data memory location that was (and still is) pointed to by array2. • Now, changing the value of an element in array2 will change that same element in array1, or visa-versa - this makes sense since both array Objects point to the same set of data values in memory! CISC124 - Prof. McLeod

  8. Aliasing Objects • Passing an Object into a method results in the method’s parameter being aliased to the Object passed. • Called “Passing by Reference”! CISC124 - Prof. McLeod

  9. Passing Parameters by Reference • For example, in main: int[] arrayA = {1, 2, 3, 4, 5}; passArray(arrayA); // invoke passArray • The passArray method: public static void passArray(int[] arrayB) { // arrayB is aliased to arrayA from main // making elemental changes to arrayB will // also change elements in arrayA in main arrayB[3] = 400; } // end passArray // arrayA[3] is 400 in main CISC124 - Prof. McLeod

  10. Passing Parameters by Reference, Cont. • The rule for parameter passing into methods is: • Objects are passed by reference, primitive types are passed by value. • See PassingDemo.java • Has a method with two parameters - an array and an int - which one(s) will stay changed? • Instead of going element by element, if you re-assign the array to another array within the method, what happens? • Does this rule apply to Strings, as well? CISC124 - Prof. McLeod

  11. Passing Arrays by Reference • Summary of PassingDemo.java: • Primitive types are passed by value. • Only element by element changes in arrays will “stick”. • Re-assigning the array to a pointer that has local scope in a method will not “stick”. • If you make element by element changes using an aliased local pointer (like the parameter), changes will “stick”. • Strings are immutable, so this does not apply. You cannot make elemental changes inside a String, even though a String is passed by reference. CISC124 - Prof. McLeod

  12. Passing Arrays by Reference, Cont. • So, mutable Objects (like arrays) can be passed into and out of a method through the parameter list. If a method changes the contents of a mutable Object passed into it – those changes “stick” even when the method is complete. CISC124 - Prof. McLeod

  13. Aside - Comparing Objects • Testing arrays and Objects for equality (with the “==“ boolean operator) is also interesting: • This test will only give a true when both objects have been aliased, using the assignment operator “=“. • So, even if both arrays have identical contents, “==“ will return false, unless both arrays point to the same location. • This means that comparing Objects with “==“ only compares pointers, not contents. CISC124 - Prof. McLeod

  14. Pointers – A Question • So, which way is better to declare a 3 by 10000 two-dimensional array?: int[][] wayOne = new int[3][10000]; int[][] wayTwo = new int[10000][3]; • Or, it makes no difference? CISC124 - Prof. McLeod

  15. How Much Memory Does a Pointer Use? String aString = “Hello!”; • How much memory does aString consume? Not the string itself, just the pointer to the string. • 32 bits for 32 bit Java, 64 bits for 64 bit Java (unless you are using compressed pointers…) • Where is the type information for the pointer contained? (Used by instanceof) CISC124 - Prof. McLeod

  16. null Pointer or null Reference • null is not a keyword in Java – more like a literal constant. • What is a null pointer? • What is a null pointer error? • Does null have a type? • Can you test a pointer to see if it is null? How? • Why would you want to? CISC124 - Prof. McLeod

  17. null Pointer or null Reference, Cont. • What is the difference between string1, string2 and string3 ?: String string1 = ""; String string2 = null; String string3; • See TestNull.java. • Is an unassigned primitive type variable null? • What are the contents of an uninitialized array? CISC124 - Prof. McLeod

  18. null References, Cont. • The idea of a null reference was first introduced into ALGOL W back in 1965 by C.A.R. Hoare (also known as the inventor of Quicksort). • See: http://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare CISC124 - Prof. McLeod

  19. Using null in Java • You can test to see if a pointer is null (See file input code, for example. This is how you detect the end of the file.) • Sometimes, to widen the scope of a variable you need to declare it before you can instantiate it. In this case you must assign the variable pointer to null. “Bad things” will happen if you do not! • A NullPointerException is probably the most frustrating error to encounter in Java! CISC124 - Prof. McLeod

More Related