Principles of Software Construction: Objects, Design, and - - PowerPoint PPT Presentation
Principles of Software Construction: Objects, Design, and Concurrency Exceptions and contracts in Java Josh Bloch Charlie Garrod 17-214 1 Administrivia Homework 1 due Today 11:59 p.m. Everyone must read and sign our collaboration
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FileInputStream fis = new FileInputStream(fileName); if (fis == null) { switch (errno) { case _ENOFILE: System.err.println("File not found: " + …); return -1; default: System.err.println("Something else bad happened: " + …); return -1; } } DataInput dataInput = new DataInputStream(fis); if (dataInput == null) { System.err.println("Unknown internal error."); return -1; // errno > 0 set by new DataInputStream } int i = dataInput.readInt(); if (errno > 0) { System.err.println("Error reading binary data from file"); return -1; } // The Slide lacks space to close the file. Oh well. return i;
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FileInputStream fis = new FileInputStream(fileName); if (fis == null) { switch (errno) { case _ENOFILE: System.err.println(“File not found: “ + …); return -1; default: System.err.println(“Something else bad happened: “ + …); return -1; } } DataInput dataInput = new DataInputStream(fis); if (dataInput == null) { System.err.println(“Unknown internal error.”); return -1; // errno > 0 set by new DataInputStream } int i = dataInput.readInt(); if (errno > 0) { System.err.println(“Error reading binary data from file”); return -1; } // The Slide lacks space to close the file. Oh well. return i;
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FileInputStream fileInput = null; try { fileInput = new FileInputStream(fileName); DataInputStream dataInput = new DataInputStream(fileInput); return dataInput.readInt(); } catch (IOException e) { System.err.println("Could not read int from file: " + e); return DEFAULT_VALUE; }
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public static void main(String[] args) { try { test(); } catch (ArrayIndexOutOfBoundsException e) { System.out.println"("Caught index out of bounds exception: " + e); } } public static void test() { try { System.out.println("Top"); int[] a = new int[10]; a[42] = 42; // Index is too high; throws exception System.out.println("Bottom"); } catch (NegativeArraySizeException e) { System.out.println("Caught negative array size exception: " + e); } }
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… … … … Checked Exceptions
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// Empty catch block IGNORES exception – Bad smell in code! try { ... } catch (SomeException e) { }
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FileInputStream fileInput = null; try { fileInput = new FileInputStream(fileName); DataInputStream dataInput = new DataInputStream(fileInput); return dataInput.readInt(); } catch (IOException e) { System.err.println("Could not read int from file: " + e); return DEFAULT_VALUE; }
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FileInputStream fileInput = null; try { fileInput = new FileInputStream(fileName); DataInputStream dataInput = new DataInputStream(fileInput); return dataInput.readInt(); } catch (IOException e) { System.err.println("Could not read int from file: " + e); return DEFAULT_VALUE; } finally { // Close file if it’s open if (fileInput != null) { try { fileInput.close(); } catch (IOException ignored) { // No recovery necessary (or possible) } } }
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try (DataInputStream dataInput = new DataInputStream(new FileInputStream(fileName))) { return dataInput.readInt(); } catch (IOException e) { System.err.println("Could not read file: " + e); return DEFAULT_VALUE; }
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static void copy(String src, String dest) throws IOException { InputStream in = new FileInputStream(src); try { OutputStream out = new FileOutputStream(dest); try { byte[] buf = new byte[8 * 1024]; int n; while ((n = in.read(buf)) >= 0)
} finally { if (out != null) out.close(); } } finally { if (in != null) in.close(); } } }
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static void copy(String src, String dest) throws IOException { try (InputStream in = new FileInputStream(src); OutputStream out = new FileOutputStream(dest)) { byte[] buf = new byte[8 * 1024]; int n; while ((n = in.read(buf)) >= 0)
} }
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/*@ requires array != null; @ @ ensures \result == @ (\sum int j; 0 <= j && j < array.length; array[j]); @*/ int total(int array[]);
postcondition precondition
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/** * Returns the element at the specified position of this list. * * <p>This method is <i>not</i> guaranteed to run in constant time. * In some implementations, it may run in time proportional to the * element position. * * @param index position of element to return; * must be non-negative and less than the size of this list. * @return the element at the specified position of this list * @throws IndexOutOfBoundsException if the index is out of range * ({@code index < 0 || index >= this.size()}) */ E get(int index);
postcondition precondition exceptional behavior
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final class PhoneNumber { private final short areaCode; private final short prefix; private final short lineNumber; ... @Override public String toString() { return String.format("(%03d) %03d-%04d", areaCode, prefix, lineNumber); } } PhoneNumber jenny = ...; System.out.println(jenny); Prints: (707) 867-5309
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public final class PhoneNumber { private final short areaCode; private final short prefix; private final short lineNumber; @Override public boolean equals(Object o) { if (!(o instanceof PhoneNumber)) // Does null check return false; PhoneNumber pn = (PhoneNumber) o; return pn.lineNumber == lineNumber && pn.prefix == prefix && pn.areaCode == areaCode; } ... }
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Whenever it is invoked on the same object more than once during an execution
same integer, provided no information used in equals comparisons on the
– If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. – It is not required that if two objects are unequal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.
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public final class PhoneNumber { private final short areaCode; private final short prefix; private final short lineNumber; @Override public int hashCode() { int result = 17; // Nonzero is good result = 31 * result + areaCode; // Constant must be odd result = 31 * result + prefix; // " " " " result = 31 * result + lineNumber; // " " " " return result; } ... }
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public final class PhoneNumber { private final short areaCode; private final short prefix; private final short lineNumber; @Override public int hashCode() { return Objects.hash(areaCode, prefix, lineNumber); } ... }
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