Java Magazine - May/June 2012

//java architect /

programs contained missing dependencies or referenced old repositories. Our final selection includes software written both before and after the introduction of generics and comprises more than

5 million lines of code.

We customized the Java compiler that is freely available as part of the OpenJDK, modifying the type checker to make arrays invariant. In this way, we could simply use the compiler to identify any uses of covariant arrays in our code corpus, because these uses would be reported as compile errors.

This required changing the method isSubtypeUnchecked(Type t, Type s, Warner warn), which is called by the compiler to check whether two types are subtypes (that is, whether or not one type is sub-stitutable for the other). Whenever this method was called with array types, we required the types of the two arrays to be identical:

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if (t.tag == ARRAY && s.tag == ARRAY) { }

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As a result, uses of covariant arrays are reported as a compile error. For example, the code in Listing 1 returns the following two errors:

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Main.java:5:incompatible types found: java.lang.String[] required: &java.lang.Object[] Object[]=new String[ 10]; Main.java:10:incompatible types found: java.lang.String[]

required: java.lang.Object[] return new String[ 10];

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11%

Method call Constructor call Assignment Return statement

78%

Figure 1

16%

User-defined Library

84%

Figure 2

Results of the Study

We ran the modified Java compiler over our corpus and discovered 324 uses of covariant arrays in 9 out of 64 programs. This is a surprisingly low number considering that our corpus exceeds

5 million lines of code, which equates to one covariant array use for every 15,000 lines of code. This indicates that covariant arrays are used very infrequently.

Figure 1 shows the contexts in which covariant use of arrays occurred. The vast majority of these uses (89 percent) occurred when a method or constructor was called with a subtype of the required array parameter type. For example, if we called our find(Object item, Object[] arr) method from above with an argument of type String[], this would be classified as a covariant method call.