Java

A boxed primitive is created just to call its compareTo method. It's more efficient to use the associated static compare method (for double and float since Java 1.4, for other primitive types since Java 7) which works on primitives directly.

This method or field is or uses a Map or Set of URLs. Since both the equals and hashCode method of URL perform domain name resolution, this can result in a big performance hit.

This method accesses the value of a Map entry, using a key that was retrieved from a keySet iterator. It is more efficient to use an iterator on the entrySet of the map, to avoid the Map.get(key) lookup.

The method calls Connection.prepareStatement inside the loop passing the constant arguments. If the PreparedStatement should be executed several times there's no reason to recreate it for each loop iteration.

Calling String.toString is a redundant operation. Just use the string directly.

Creating new instances of java.lang.Boolean wastes memory, since Boolean objects are immutable and there are only two useful values of this type.

Using Integer's default constructor is guaranteed to always result in a new object whereas Integer.valueOf allows the compiler/class library/JVM to cache values, which is known as interning.

Using Float or Double's default constructor is guaranteed to always result in a new object whereas the valueOf method of these classes allows the JVM to cache values, which is known as interning.

Creating a new java.lang.String object using the default constructor wastes memory because the object so created will be functionally indistinguishable from the empty string constant "".

Creating a String using object creation wastes memory because the new String object so constructed will be functionally indistinguishable from the String value passed as a parameter. Just use the string directly.

This code explicitly invokes garbage collection via System.gc or Runtime.gc. Except for specific use in benchmarking, this is very dubious.

This method or field is or uses a Map or Set of URLs. Since both the equals and hashCode method of URL perform domain name resolution, this can result in a big performance hit.

Creating a String using object creation wastes memory because the new String object so constructed will be functionally indistinguishable from the String value passed as a parameter. Just use the string directly.

Creating a new java.lang.String object using the default constructor wastes memory because the object so created will be functionally indistinguishable from the empty string constant "".

Calling String.toString is a redundant operation. Just use the string directly.

This code explicitly invokes garbage collection via System.gc() or Runtime.gc(). Except for specific use in benchmarking, this is very dubious.

Creating new instances of java.lang.Boolean wastes memory, since Boolean objects are immutable and there are only two useful values of this type.

Using Integer's default constructor is guaranteed to always result in a new object whereas Integer.valueOf allows the compiler/class library/JVM to cache values, which is known as interning.

Using Float or Double's default constructor is guaranteed to always result in a new object whereas the valueOf method of these classes allows the JVM to cache values, which is known as interning.

This method calls Pattern.compile() inside a loop with constant arguments. If this Pattern will be used several times, there's no reason to compile it on each loop iteration.