15: COLLECTIONS AND MAPS in Java

Generator QR in Java 15: COLLECTIONS AND MAPS
CHAPTER 15: COLLECTIONS AND MAPS
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In many cases, it is preferable to use the instanceof operator It always returns false if its left operand is null:
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if (!(argumentObj instanceof MyRefType)) return false;
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This test has the added advantage that if the condition fails, the argument reference can be safely downcast
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Example 154 Implementing the equals() Method public class UsableVNO { // Overrides equals(), but not hashCode() private int release; private int revision; private int patch; public UsableVNO(int release, int revision, int patch) { thisrelease = release; thisrevision = revision; thispatch = patch; } public String toString() { return "(" + release + "" + revision + "" + patch + ")"; } public boolean equals(Object obj) { if (obj == this) return true; if (!(obj instanceof UsableVNO)) return false; UsableVNO vno = (UsableVNO) obj; return vnopatch == thispatch && vnorevision == thisrevision && vnorelease == thisrelease; } } // (1) // (2) // (3) // (4) // (5)
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Example 154 shows an implementation of the equals() method for version numbers Next, we provide a checklist for implementing the equals() method
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Method Overriding signature
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The method header is
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public boolean equals(Object obj) // (1)
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The signature of the method requires that the argument passed is of the type Object The following header will overload the method, not override it:
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public boolean equals(MyRefType obj) // Overloaded
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151: COMPARING OBJECTS
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The compiler will not complain Calls to overloaded methods are resolved at compile time, depending on the type of the argument Calls to overridden methods are resolved at runtime, depending on the type of the actual object referenced by the argument Comparing the objects of the class MyRefType that overloads the equals() method for equivalence, can give inconsistent results:
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MyRefType ref1 = new MyRefType(); MyRefType ref2 = new MyRefType(); Object ref3 = ref2; boolean b1 = ref1equals(ref2); boolean b2 = ref1equals(ref3);
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// True Calls equals() in MyRefType // Always false Calls equals() in Object
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However, if the equals() method is overridden correctly, only the overriding method in MyRefType is called A class can provide both implementations, but the equals() methods must be consistent
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Reflexivity Test
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This is usually the first test performed in the equals() method, avoiding further computation if the test is true The equals() method in Example 154 does this test at (2)
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Correct Argument Type
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The equals() method in Example 154 checks the type of the argument object at (3), using the instanceof operator:
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if (!(obj instanceof UsableVNO)) return false; // (3)
This code also does the null comparison correctly, returning false if the argument obj has the value null The instanceof operator will also return true if the argument obj denotes a subclass object of the class UsableVNO If the class is final, this issue does not arise there are no subclass objects The test at (3) can also be replaced by the following code in order to exclude all other objects, including subclass objects:
if ((obj == null) || (objgetClass() != thisgetClass())) return false; // (3a)
The test in (3a) first performs the null comparison explicitly The expression (objgetClass() != thisgetClass()) determines whether the classes of the two objects have the same runtime object representing them If this is the case, the objects are instances of the same class
Argument Casting
The argument is only cast after checking that the cast will be successful The instanceof operator ensures the validity of the cast, as done in Example 154 The argument is cast at (4) to allow for class-specific field comparisons:
UsableVNO vno = (UsableVNO) obj; // (4)
CHAPTER 15: COLLECTIONS AND MAPS
Field Comparisons
Equivalence comparison involves comparing certain fields from both objects to determine if their logical states match For fields that are of primitive data types, their primitive values can be compared Instances of the class UsableVNO in Example 154 have fields of primitive data types only Values of corresponding fields are compared to test for equality between two UsableVNO objects:
return vnopatch == thispatch && vnorevision == thisrevision && vnorelease == thisrelease; // (5)
If all field comparisons evaluate to true, the equals() method returns true For fields that are references, the objects referenced by the references can be compared For example, if the UsableVNO class declares a field called productInfo, which is a reference, the following code could be used:
(vnoproductInfo == thisproductInfo || (thisproductInfo != null && thisproductInfoequals(vnoproductInfo)))
The expression vnoproductInfo == thisproductInfo checks for the possibility that the two objects being compared have a common object referenced by both productInfo references In order to avoid a NullPointerException being thrown, the equals() method is not invoked if the thisproductInfo reference is null Exact comparison of floating-point values should not be done directly on the values, but on the integer values obtained from their bit patterns (see static methods FloatfloatToIntBits() and DoubledoubleToLongBits() in the Java Standard Library) This technique eliminates certain anomalies in floating-point comparisons that involve a NaN value or a negative zero (see also the equals() method in Float and Double classes) Only fields that have significance for the equivalence relation should be considered Derived fields, whose computation is dependent on other field values in the object, might be redundant to include, including only the derived fields may be prudent Computing the equivalence relation should be deterministic, therefore, the equals() method should not depend on unreliable resources, such as network access The order in which the comparisons of the significant fields are carried out can influence the performance of the equals comparison Fields that are most likely to differ should be compared as early as possible in order to short-circuit the computation In our example, patch numbers evolve faster than revision numbers, which, in turn, evolve faster than release numbers This order is reflected in the return statement at (5) in Example 154 Above all, an implementation of the equals() method must ensure that the equivalence relation is fulfilled Example 155 is a client that uses the class UsableVNO from Example 154 This client runs the same tests as the client in Example 153 The difference is that the class UsableVNO overrides the equals() method