We are really grateful to the students and teachers who have immensely appreciated the previous editions of the book and encouraged us to bring out this new edition of Understanding computer Applications with BlueJ. This book, in its ninth edition, has been thoroughly revised as per the latest ICSE curriculum. The main thrust of the book is to deal with Object Oriented Programming in Java with BlueJ. The first chapter recapitulates the entire syllabus of Class IX in concise form. It is advised to refer ‘Understanding Computer Applications with BlueJ’, Class IX book for comprehensive understanding of the concepts. The character and string manipulations have been designed separately along with the inclusion of Double Dimensional Array (DA). Each chapter contains a number of review questions and exercises which are carefully designed to provide insight into the subject. Strenuous efforts have been made to avoid errors specially programming errors. The programming examples have also been executed and verified on the computer. In spite of this, some errors may still be there. Students and readers are requested to bring the same to our knowledge. Miscellaneous programs and projects work for internal assessment have also been included in the book. Answers to objective type and very short answer type questions of each chapter have been given at the end of the book.