Difference between revisions of "Fix Code Sample"
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|Problem= Identify the problems in the code sample below. Fix the errors so that the code works correctly. | |Problem= Identify the problems in the code sample below. Fix the errors so that the code works correctly. | ||
<BR> | <BR> | ||
− | + | {{CodeBlock | |
− | + | |Code= | |
+ | public class MyClass | ||
+ | { | ||
+ | private static int numInst = 0; | ||
+ | int a; | ||
+ | double b; | ||
+ | |||
+ | public static MyClass( double b ) | ||
+ | { | ||
+ | this.a = 5; | ||
+ | this.b = b; | ||
+ | numInst++; | ||
+ | } | ||
+ | |||
+ | public static double average( int n1, int n2, int b ) | ||
+ | { | ||
+ | int total; | ||
+ | |||
+ | total = n1 + n2 + this.b; | ||
+ | |||
+ | return (double)(total / 3); | ||
+ | } | ||
+ | |||
+ | public static int getNumInst() | ||
+ | { | ||
+ | return this.numInst; | ||
+ | } | ||
+ | |||
+ | public static double square( double ) | ||
+ | { | ||
+ | return a*a; | ||
+ | } | ||
+ | |||
+ | public static double cube( double a ) | ||
+ | { | ||
+ | return a*getA()*getA(); | ||
+ | } | ||
+ | |||
+ | public void setNumInst( int newSum ) | ||
+ | { | ||
+ | numInst = newSum; | ||
+ | } | ||
+ | |||
+ | public static void setA( int newA ) | ||
+ | { | ||
+ | a = newA; | ||
+ | } | ||
+ | |||
+ | public double getA() | ||
+ | { | ||
+ | return this.a; | ||
+ | } | ||
+ | } | ||
+ | }} | ||
+ | |||
+ | |SideSectionTitle=...By Students | ||
|SideSection= | |SideSection= | ||
− | + | "I can recall seeing many of the errors here in the code I wrote during my first year in Computer Science and not having a clue | |
− | <BR> | + | what was causing them. Something like this wiki page would have saved me a lot of time spent debuggin simple errors. |
− | + | <BR><BR> | |
+ | It is important to understand exactly what your code is doing when you write and to pay attention to detail, as this example shows, even a | ||
+ | small typo can create an error which prevents your program from compiling. | ||
+ | It is also very important to debug your code thoroughly to reveal any tricky errors." | ||
+ | |||
+ | |Solution=The first problem with this code is the static constructor. Since this needs to access instance variables, | ||
+ | the method must be a instance method. | ||
+ | {{CodeBlock | ||
+ | |Code= | ||
+ | public MyClass( double b ) | ||
+ | { | ||
+ | this.a = 5; | ||
+ | this.b = b; | ||
+ | numInst++; | ||
+ | } | ||
+ | }} | ||
+ | |||
+ | The next problem is that our average() method is referencing the instance variable <b>b</b>, rather than the <b>int b</b> passed in as | ||
+ | a parameter. To solve this, we just need to remove the <b>this.</b> in front of the reference to <b>b</b>. | ||
+ | {{CodeBlock | ||
+ | |Code= | ||
+ | total = n1 + n2 + b; | ||
+ | }} | ||
+ | |||
+ | The getNumInst() method has a similar problem, it tries to reference <b>this</b>, which is not allowed in a static method. Since numInst is a static | ||
+ | variable, we can solve this as above, simply removing the reference to <b>this</b>. | ||
+ | {{CodeBlock | ||
+ | |Code= | ||
+ | return numInst; | ||
+ | }} | ||
+ | |||
+ | The are two problems with our square() method. First, there is no identifier for the parameter passed in. Second, we are referencing an instance variable from | ||
+ | a static method. The solution to both problems is to add <b>a</b> as an identifier in the parameter list. | ||
+ | {{CodeBlock | ||
+ | |Code= | ||
+ | public double square( double a ) | ||
+ | }} | ||
+ | |||
+ | The problem with cube() is that is calls an instance method from a static method. This is not allowed. Replace the calls to getA() with one call to | ||
+ | square(a). | ||
+ | {{CodeBlock | ||
+ | |Code= | ||
+ | return a*square(a); | ||
+ | }} | ||
+ | |||
+ | The setNumInst() method is correct as is, we don't have to make any changes to it. | ||
+ | |||
+ | Although there is no reference to <b>this</b>, the variable <b>a</b> referenced by setA() is an instance variable and cannot be called from a static method. | ||
+ | To fix this, remove the <b>static</b> from the method signature. | ||
+ | {{CodeBlock | ||
+ | |Code= | ||
+ | public void setA( int newA ) | ||
+ | }} | ||
+ | |||
+ | The getA() method works correctly and needs no changes. | ||
− | + | |SolutionCode=public class MyClass | |
+ | { | ||
+ | private static int numInst = 0; | ||
+ | int a; | ||
+ | double b; | ||
+ | |||
+ | public MyClass( double b ) | ||
+ | { | ||
+ | this.a = 5; | ||
+ | this.b = b; | ||
+ | numInst++; | ||
+ | } | ||
+ | |||
+ | public static double average( int n1, int n2, int b ) | ||
+ | { | ||
+ | int total; | ||
+ | |||
+ | total = n1 + n2 + b; | ||
+ | |||
+ | return (double)(total / 3); | ||
+ | } | ||
+ | |||
+ | public static int getNumInst() | ||
+ | { | ||
+ | return numInst; | ||
+ | } | ||
+ | |||
+ | public static double square( double a ) | ||
+ | { | ||
+ | return a*a; | ||
+ | } | ||
− | + | public static double cube( double a ) | |
− | + | { | |
− | + | return a*square(a); | |
+ | } | ||
+ | |||
+ | public void setNumInst( int newSum ) | ||
+ | { | ||
+ | numInst = newSum; | ||
+ | } | ||
+ | |||
+ | public void setA( int newA ) | ||
+ | { | ||
+ | a = newA; | ||
+ | } | ||
+ | public double getA() | ||
+ | { | ||
+ | return this.a; | ||
+ | } | ||
+ | } | ||
}} | }} |
Latest revision as of 19:34, 25 June 2012
Back to the Program-A-Day homepage
ProblemIdentify the problems in the code sample below. Fix the errors so that the code works correctly.
public class MyClass { private static int numInst = 0; int a; double b; public static MyClass( double b ) { this.a = 5; this.b = b; numInst++; } public static double average( int n1, int n2, int b ) { int total; total = n1 + n2 + this.b; return (double)(total / 3); } public static int getNumInst() { return this.numInst; } public static double square( double ) { return a*a; } public static double cube( double a ) { return a*getA()*getA(); } public void setNumInst( int newSum ) { numInst = newSum; } public static void setA( int newA ) { a = newA; } public double getA() { return this.a; } } |
...By Students"I can recall seeing many of the errors here in the code I wrote during my first year in Computer Science and not having a clue
what was causing them. Something like this wiki page would have saved me a lot of time spent debuggin simple errors.
| |
---|---|---|
SolutionThe first problem with this code is the static constructor. Since this needs to access instance variables, the method must be a instance method. public MyClass( double b ) { this.a = 5; this.b = b; numInst++; } The next problem is that our average() method is referencing the instance variable b, rather than the int b passed in as a parameter. To solve this, we just need to remove the this. in front of the reference to b. total = n1 + n2 + b; The getNumInst() method has a similar problem, it tries to reference this, which is not allowed in a static method. Since numInst is a static variable, we can solve this as above, simply removing the reference to this. return numInst; The are two problems with our square() method. First, there is no identifier for the parameter passed in. Second, we are referencing an instance variable from a static method. The solution to both problems is to add a as an identifier in the parameter list. public double square( double a ) The problem with cube() is that is calls an instance method from a static method. This is not allowed. Replace the calls to getA() with one call to square(a). return a*square(a); The setNumInst() method is correct as is, we don't have to make any changes to it. Although there is no reference to this, the variable a referenced by setA() is an instance variable and cannot be called from a static method. To fix this, remove the static from the method signature. public void setA( int newA ) The getA() method works correctly and needs no changes. | ||
Code |