free visual basic barcode generator A Closer Look at Methods and Classes in Java

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A Closer Look at Methods and Classes
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/* This program demonstrates the difference between public and private */ class Test { int a; // default access public int b; // public access private int c; // private access // methods to access c void setc(int i) { // set c's value c = i; } int getc() { // get c's value return c; } } class AccessTest { public static void main(String args[]) { Test ob = new Test(); // These are OK, a and b may be accessed directly oba = 10; obb = 20; // This is not OK and will cause an error obc = 100; // Error! // You must access c through its methods obsetc(100); // OK Systemoutprintln("a, b, and c: " + oba + " " + obb + " " + obgetc()); } }
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As you can see, inside the Test class, a uses default access, which for this example is the same as specifying public b is explicitly specified as public Member c is given private access This means that it cannot be accessed by code outside of its class So, inside the AccessTest class, c cannot be used directly It must be accessed through its public methods: setc( ) and getc( ) If you were to remove the comment symbol from the beginning of the following line,
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// obc = 100; // Error!
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then you would not be able to compile this program because of the access violation To see how access control can be applied to a more practical example, consider the following improved version of the Stack class shown at the end of 6
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// This class defines an integer stack that can hold 10 values class Stack { /* Now, both stck and tos are private This means
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that they cannot be accidentally or maliciously altered in a way that would be harmful to the stack */ private int stck[] = new int[10]; private int tos; // Initialize top-of-stack Stack() { tos = -1; } // Push an item onto the stack void push(int item) { if(tos==9) Systemoutprintln("Stack is full"); else stck[++tos] = item; } // Pop an item from the stack int pop() { if(tos < 0) { Systemoutprintln("Stack underflow"); return 0; } else return stck[tos--]; } }
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As you can see, now both stck, which holds the stack, and tos, which is the index of the top of the stack, are specified as private This means that they cannot be accessed or altered except through push( ) and pop( ) Making tos private, for example, prevents other parts of your program from inadvertently setting it to a value that is beyond the end of the stck array The following program demonstrates the improved Stack class Try removing the commented-out lines to prove to yourself that the stck and tos members are, indeed, inaccessible
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class TestStack { public static void main(String args[]) { Stack mystack1 = new Stack(); Stack mystack2 = new Stack(); // push some numbers onto the stack for(int i=0; i<10; i++) mystack1push(i); for(int i=10; i<20; i++) mystack2push(i); // pop those numbers off the stack Systemoutprintln("Stack in mystack1:"); for(int i=0; i<10; i++) Systemoutprintln(mystack1pop()); Systemoutprintln("Stack in mystack2:");
7:
A Closer Look at Methods and Classes
for(int i=0; i<10; i++) Systemoutprintln(mystack2pop()); // these statements are not legal // mystack1tos = -2; // mystack2stck[3] = 100; } }
Although methods will usually provide access to the data defined by a class, this does not always have to be the case It is perfectly proper to allow an instance variable to be public when there is good reason to do so For example, most of the simple classes in this book were created with little concern about controlling access to instance variables for the sake of simplicity However, in most real-world classes, you will need to allow operations on data only through methods The next chapter will return to the topic of access control As you will see, it is particularly important when inheritance is involved
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