Reflection.md

{ StringWriter writer = new StringWriter(); PrintStream out = new PrintStream(new WriterOutputStream(writer));

    // when
    setStaticField(System.class, "out", out);

}

private void setStaticField(Class<?> clazz, String fieldName, Object value) { try{ Field field = clazz.getDeclaredField(fieldName); field.setAccessible(true);

        Field modifiersField = Field.class.getDeclaredField( "modifiers" );
        modifiersField.setAccessible( true );
        modifiersField.setInt( field, field.getModifiers() & ~Modifier.FINAL );

        field.set(clazz, value);
    }catch(NoSuchFieldException ex){
        throw new RuntimeException("can't set value");
    } catch (IllegalAccessException e) {
        throw new RuntimeException("can't set value", e);
    }
}