primitive-boolean To String concatenation/conversion

The exact rules are spelled out in the Java Language Specification, §5.1.11. String Conversion

According to those rules, "str" + bool is equivalent to:

"str" + new Boolean(bool).toString()

That said, the compiler is permitted considerable leeway in how exactly the overall expression is evaluated. From JLS §15.18.1. String Concatenation Operator +:

An implementation may choose to perform conversion and concatenation in one step to avoid creating and then discarding an intermediate String object. To increase the performance of repeated string concatenation, a Java compiler may use the StringBuffer class or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.

For primitive types, an implementation may also optimize away the creation of a wrapper object by converting directly from a primitive type to a string.

For example, with my compiler the following:

boolean bool = true;
System.out.println("the value of bool is : " + bool);

is exactly equivalent to:

boolean bool = true;
System.out.println(new StringBuilder("the value of bool is : ").append(bool).toString());

They result in identical bytecodes:

Code:
   0: iconst_1      
   1: istore_1      
   2: getstatic     #59                 // Field java/lang/System.out:Ljava/io/PrintStream;
   5: new           #166                // class java/lang/StringBuilder
   8: dup           
   9: ldc           #168                // String the value of bool is : 
  11: invokespecial #170                // Method java/lang