![Javier Gonzalez-Sanchez | CSE110 | Summer 2017 | 7
Arrays of Objects
• The elements of an array can be object references.
• Suppose that we have the Student class
Student[] list = new Student[5];
//The following causes NullPointerException
int id = list[0].getID();
//We need to instantiate each object first:
list[0] = new Student(“Bob”);
list[1] = new Student(“Mary”);
list[2] = new Student(“Peter”);
// ...
// OR we can use the Initializer:
Student[] list = {new Student(“Bob”),
new Student(“Mary”),
new Student(“Peter”) };](https://image.slidesharecdn.com/cse110l24-references-170708044548/85/201707-CSE110-Lecture-24-7-320.jpg)
Uploaded byJavier Gonzalez-Sanchez
201707 CSE110 Lecture 24
This document summarizes key concepts around references, arrays of objects, and parameters from a lecture on Java programming. It discusses how assignment with primitive types copies values but assignment with object references copies memory addresses, creating aliases. It also explains how arrays can hold object references, and how objects passed as parameters are aliases, so changes within a method also change the original parameter, unless it is reinstantiated. The document is from a CSE110 Principles of Programming with Java lecture by Javier Gonzalez-Sanchez.
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201707 CSE110 Lecture 24
- 1. CSE110 Principles of Programming withJava Lecture 24: References, Arrays of Objects, and Parameters Javier Gonzalez-Sanchez javiergs@asu.edu javiergs.engineering.asu.edu Office Hours: By appointment
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- 3. JavierGonzalez-Sanchez|CSE110|Summer2017|3 Assignment Revisited • Theact of assignment takes a copy of a value and stores it in a variable • For primitive types:
- 4. Javier Gonzalez-Sanchez |CSE110 | Summer 2017 | 4 Reference Assignment • An object reference variable holds the memory address of an object • Rather than dealing with arbitrary addresses, we often depict a reference graphically as a “pointer” to an object • For object references, assignment copies the memory location: jane john jane john
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- 7. Javier Gonzalez-Sanchez |CSE110 | Summer 2017 | 7 Arrays of Objects • The elements of an array can be object references. • Suppose that we have the Student class Student[] list = new Student[5]; //The following causes NullPointerException int id = list[0].getID(); //We need to instantiate each object first: list[0] = new Student(“Bob”); list[1] = new Student(“Mary”); list[2] = new Student(“Peter”); // ... // OR we can use the Initializer: Student[] list = {new Student(“Bob”), new Student(“Mary”), new Student(“Peter”) };
- 8. Javier Gonzalez-Sanchez |CSE110 | Summer 2017 | 8 Arrays of Objects
- 9. Javier Gonzalez-Sanchez |CSE110 | Summer 2017 | 9 Arrays of Objects
- 10. Javier Gonzalez-Sanchez |CSE110 | Summer 2017 | 10 Arrays of Objects
- 11. Javier Gonzalez-Sanchez |CSE110 | Summer 2017 | 11 Arrays of Objects
- 12.
- 13. JavierGonzalez-Sanchez|CSE110|Summer2017|13 Objects as Parameters •Parameters in a Java method are passed by value • This means that a copy of the actual parameter (the value passed in) is stored into the formal parameter (in the method header) • Passing parameters is therefore similar to an assignment statement • When an object is passed as a parameter to a method, the actual parameter and the formal parameter become aliases of each other
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- 15. Javier Gonzalez-Sanchez |CSE110 | Summer 2017 | 15 Objects as Parameters
- 16. JavierGonzalez-Sanchez|CSE110|Summer2017|16 Objects as Parameters •Since a1 is a primitive data, any change that was made to f1 in the method changeValues does not affects a1 • The object a2 was assigned to the value “222” and when a2 was passed to changeValues method, a2 and f2 became aliases so the change made to f2 will be seen in a2 in the main method. • The object a3 was assigned to the value “333” and when a3 was passed to changeValues method, a3 and f3 became aliases. However when f3 was re-instantiated with the reserved word new, f3 is given a different address from the one a3 has, thus any change made to f3 after that point will not affect a3 in the main method.
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- 18. CSE110 - Principlesof Programming Javier Gonzalez-Sanchez javiergs@asu.edu Summer 2017 Disclaimer. These slides can only be used as study material for the class CSE110 at ASU. They cannot be distributed or used for another purpose.