Debugging Given a failure, find and fix the error. Key is - - PowerPoint PPT Presentation

Debugging

Given a failure, find and fix the error. Key is reproducibility What input causes the failure?

Debugging Strategies

Goal: Find a point in your program where it behaves differently than what you expect. Work backwards to the error. assert calls Output statements within System.out.println Use a debugger.

Debugger

Set a breakpoint to pause execution. View variable values while paused. Continue execution by: Continuing to the next breakpoint Stepping to the next statement in the method Stepping into a method being called

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Breakpoints

Line of code where execution should pause so that you can look at the values of variables To set a breakpoint in Eclipse, double-click in the margin to the left of the line where you want to stop A blue dot will appear in the margin

Running the Debugger

Click on the bug icon rather than the play button Execution pauses when reach breakpoint, showing an arrow where the execution has stopped

Eclipse Debug Perspective

Call Stack Execution Controls Variables

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Execution Controls

Continue Stop Execution Step Into Step Over Stop when current method returns

Variables Call Stack

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Reading Stack Traces

Focus on the top

Reading Stack Traces

Where the failure occurred Nature of the failure

NullPointerException

Means a variable has been used before it was set.

private ImageIcon snow;

• public SnowFlake(ImageIcon snow, int left, int screenHeight,

int fallOffset) { super(-snow.getIconHeight(), screenHeight, fallOffset); this.left = left; this.top = -snow.getIconHeight(); }

• public void paint (JComponent component, Graphics g) {

snow.paintIcon(component, g, left, top); }

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NullPointerException

Means a variable has been used before it was set.

private ImageIcon snow;

• public SnowFlake(ImageIcon snowIcon, int left, int screenHeight,

int fallOffset) { super(-snow.getIconHeight(), screenHeight, fallOffset); ImageIcon snow = snowIcon; this.left = left; this.top = -snow.getIconHeight(); }

• public void paint (JComponent component, Graphics g) {

snow.paintIcon(component, g, left, top); }

Another Stack Trace

Notice that your method is not at the top of the stack. Means you are passing null as a parameter value snow.paintIcon(component, g, left, top); Either component or g must be null.

Other Common Exceptions

ArrayIndexOutOfBoundsException - negative value, or value too big ClassCastException - attempting to cast to something of the wrong type Polygon p = new Rectangle(); Rectangle r = (Rectangle) p; Circle e = (Circle) p;

OK ClassCastException

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