1- "The Court notes that in a letter written on 12 June 1953 to the British Adviser to the Sultan of Johor, the Colonial Secretary of Singapore asked for information about the status of Pedra Branca/Pulau Batu Puteh in the context of determining the boundaries of the “Colony’s territorial waters”. In a letter dated 21 September 1953, the Acting State Secretary of Johor replied that the “Johore Government [did] not claim ownership” of the island. The Court considers that this correspondence and its interpretation are of central importance “for determining the developing understanding of the two Parties about sovereignty over Pedra Branca/Pulau Batu Puteh” and finds that the Johor’s reply shows that as of 1953 Johor understood that it did not have sovereignty over Pedra Branca/Pulau Batu Puteh."
2- "The Court finally examines the conduct of the Parties after 1953 with respect to the island. Having reviewed all arguments submitted to it, it finds that certain acts, inter alia the investigation of shipwrecks by Singapore within the island’s territorial waters and the permission granted or not granted by Singapore to Malaysian officials to survey the waters surrounding the island, may be seen as conduct à titre de souverain. The Court also considers that some weight can be given to the conduct of the Parties in support of Singapore’s claim (i.e., the absence of reaction from Malaysia to the flying of the Singapore ensign on the island, the installation by Singapore of military communications equipment on the island in 1977, and the proposed reclamation plans by Singapore to extend the island, as well as a few specific publications and maps). "
3- "The Court concludes, especially by reference to the conduct of Singapore and its predecessors à titre de souverain, taken together with the conduct of Malaysia and its predecessors including their failure to respond to the conduct of Singapore and its predecessors, that by 1980 (when the dispute crystallized) sovereignty over Pedra Branca/Pulau Batu Puteh had passed to Singapore. The Court thus concludes that sovereignty over Pedra Branca/Pulau Batu Puteh belongs to Singapore."
You guys can check out the actualy content of the verdict as follows;
The Press Release from INTERNATIONAL COURT OF JUSTICE on the case of Sovereignty over Pedra Branca/Pulau Batu Puteh, Middle Rocks and South Ledge (Malaysia/Singapore)
And here i`ve rehighlight all the reasoning in Bahasa Melayu
1-Sepucuk surat bertarikh 12 June 1953 dari Colonial Secretary of Singapore bertanya tentang status Pulau Batu Puteh/Pedra Branca.
2-Kemudian pada 21 September 1953 Acting State Secretary of Johor membalas surat tersebut dengan mengatakan yang Kerajaan Negeri Johor tidak menuntuk hak ke atas pulau itu.
3-ketiadaan reaksi dari Malaysia ketika i) kedatangan kedatangan kapal tentera laut Singapur ii) pemasangan alat komunikasi oleh tentera Singapura pada 1977 iii) cadangan penambakan tanah oleh Singapura iii) beberapa lagi hasil cetakan dan peta yang berkaitan dengan Pulau Batu Puteh